I intend this thread to be an easy-to-find source of information on what it takes to run larger tires and wheels on our HDs. My goal is for no one who reads this to be surprised at the level of effort that is often needed to run larger tires wheels without tire rubbing or instability. I will present my experiences and add in the applicable experiences of others I have come across in this quest to run larger tires. The meat of the discussion start in the section entitled: Issues with Larger Tires and Wheels.
Disclaimers/Philosophy
Opinions on what it takes to run a larger tire/wheel vary. These opinions vary based on a driver’s beliefs, perceptiveness, experience, conditions he/she drives in, and his/her driving style. Additionally, there are some minor differences in individual trucks that have a small play as well. Before someone’s opinion can be evaluated, you need to know about their opinions, experience, driving conditions, and driving style. Common statements like “I have run 475/30/27s for 100,000 miles with no lift and have had absolutely no problem” are of limited usefulness unless you know how the truck is being used and what the driver considers acceptable.
I am not the final word on any of this. I am not a professional mechanic. I am a 46 yr old "hobbyist" mechanic with advanced do-it-yourself skills. I tore into my first vehicle 29 years ago and have maintained, repaired, upgraded, and rebuilt several diverse types of vehicles—from drag race cars to daily drivers to 4x4s—since then. I have done this work in the garages of the many houses I have lived in using hand tools, floor jacks, an ever increasing inventory of specialty tools, and basic air tools. I hope the perspective of a “do-it-your-selfer” is useful to fellow do-it-your-selfers. No doubt the professional mechanics have the highest level of skill and knowledge, but it’s probably been a while since many of them had to roll around on the garage floor under a vehicle with a center section or transmission on their chest…………….
I am very perceptive (fussy?) concerning the operation of my truck. I have been a USAF pilot for almost 23 years, and it has become part of my nature to pay close attention what is going on with any vehicle I am operating. It is rare that a noise, vibration, gauge reading, or “feel” escapes me. I think many of us are that way, but I also realize many are not. Any tire rubbing at all is unacceptable to me, and I am not likely to miss it when a wheel does rub. That is an important point to keep in mind when I talk about what I think works and what doesn’t. I drive fairly aggressively, tending to “launch” the truck from stops and tending to “slalom” when I turn corners, though I usually drive very close to the speed limit. However, here in AZ, that means 75-80 mph every day I drive to work. I bought my truck to be a 20 yr truck that could do anything and everything in a very stable and controlled manner, which includes: hauling the family, hunting on rough roads and mildly off road, hauling heavy, towing heavy, and taking me to work every day. I didn’t realize I would be slowly sucked into running more and more power…………
My Truck
I have a 2002 Chevy, crew cab, short bed, 2500 HD DMax/Alli. I bought it new in June of 2002 and currently have 60,000 miles on it. So far, the truck has taken me to work, hauled the family a fair amount, taken me hunting in rough roads/off road, hauled heavy a few times, and towed medium a couple times. I have not yet towed heavy. I lived near San Antonio the first 2.5 years I had the truck and have lived near Tucson for the last 1.5 years. I am driving five times as far to work now than I was when I lived in Texas.
On this truck I progressed from: running 265s on stock rims, to cranking t-bars w/aftermarket keys and adding Bilstein shocks, to doing all the fender mods and running the 285s on 16x8s (4.5" BS) that sometimes rubbed, to installing (myself) a 4" Rancho (with 3.5" rear lift) and running 295s, to swapping the Rancho front lift for a Tuff Country 4" front lift, to finally going to 315s which require 5 1/8" of lift in the front. This doesn't count the three sets of t-bar relocators, the three sets of t-bar keys, or the 1/2 dozen stitches I needed when a t-bar adjusted bolt stripped and launched the breaker bar into my fore head.
Issues with Larger Tires and Wheels
Please note. I am using 16” wheel size as my basis, but the same issues apply to other sizes as well. The critical factors are the tire height, tire width, and wheel backspacing.
Stock 245/75R16 (30.47” x 9.65”). The factory combination for 2500s is LT 245/75R16s on 16x6.5” wheels with 4.875” of measured backspacing. (Please note some may call that 5” of backspacing.) Some dealers are selling trucks with 265/75R16s on the factory 16x6.5” wheels—my friend just bought one so equipped. Several have posted that the 3500 SRW comes with a 16x7” wheel.
265/75R16s (31.65” x 10.43”)
I ran 265/75R16s on a bone-stock suspension with no tire clearance problems. Many others here have reported the same results. I took the truck hunting in this configuration, crammed the skid plates full of mud and vegetation, so I added aftermarket torsion bar keys, cranked the t-bars 1.75”, and added stock height Bilstein shocks with 1” spacers under the top mount. I did not need to get the aftermarket torsion bar keys to get 1.75” of lift. After I cranked the t-bars I went hunting over the same terrain and had no ground clearance issues.
265/75R16s seem to work very well on the factory wheels, even though the stock rims are 1/2” narrower than what the tire manufacturers recommend for 265/75R16s. You shouldn’t have to crank the t-bars or do any other mods to run 265/75R16s on stock wheels without rubbing.
I don’t know if you’d have to crank the t-bars to run aftermarket wheels with the 265s, assuming you are running a typical 16x8” wheel with around 4.5” of backspacing. Many guys crank the t-bars to level the truck when they install 265s on aftermarket wheels because they like the leveled look, but that doesn’t mean the t-bars must be cranked.
285/75R16 (32.83” x 11.22”)
Here is where the issues and controversies seem to be the worst. You have guys like me who insist this tire will rub without a lift and you have guys who insist they have had absolutely no rubbing of any kind. Let’s start with there is agreement. To run a 285/75R16 or similar size tire you will have to crank the torsion bars and/or work on the inner fender liners and trim the air dam of the front bumper. Everyone seems to at least do those things when running this size of tire, and that is where the agreement ends.
You will get better clearance running the 285s on stock wheels. This is because the stock wheel is narrower and has more backspacing than the typically used aftermarket wheels. The stock wheel is 6.5” wide and has a 4.875” of backspacing. This means the center line of the wheel and tire is 1.625” toward the inside of the wheel well. An 8” wide wheel with 4.5” of backspacing has its centerline .5” toward the inside of the wheel well. This extra 1.125” the 8” wheel is moved toward the outside of the fender well causes a reduction of tire clearance when turning. It causes a reduction in clearance because the tire is on a longer moment arm and moves in a larger arc, meaning it comes closer to the wheel well during turns. Again, you’ll get the better clearance running 285/75R16s on the stock wheels than you will wider wheels with less backspacing.
However, there is a potential problem running 285/75R16s on the stock 6.5” wide wheels. The tire manufacturers recommend a 7.5” minimum rim width for a 285/75R16. Why? On too narrow a rim, the tire tread tries to curve across the width vs remaining flat. If you run a tire with the tread rounded across its width, you will wear out the centers—been there, done that. In order to get the width of the tread flat you have to run less PSI. When you run less PSI the sidewalls are less stiff, meaning the tire will flex more, meaning you have less stability.
How much less stability? That is the fuzzy area. You have guys like me to whom the idea of even the potential of less stability is heinous. After all, I can’t slalom my truck around turns if the sidewalls flex too much. Further, hauling a heavy load or a heavy trailer—especially on a windy, twisty road—with sidewalls that flex too much would not be pleasant. However, if you never tow/haul heavy or slalom, then the stability of a 285 on the stock wheel probably isn’t something you’d even notice. Others here have talked about towing light to medium loads without a significant loss in stability, probably because the good all terrain tires are designed to be “aired down” to improve traction in certain off road situations. If they have strong enough sidewalls to run aired down, then chances are they will maintain decentstability at lower air pressures with moderate loads. Regardless of all that, running a 285 on a 6.5” rim will not be as stable as running a 285 on an 8” rim, and not every tire company will install tires on wheels that are that much narrower than what the tire manufacturer recommends.
No matter which wheel you run with your 285/75R16, the tight point of tire clearance is between the rear outside of the tire and the rear outside of the fender well when the steering wheel is turned about 3/4 travel. I do not have a hard and fast rule, but with the various tire sizes and lifts I have run, it seems that about 1” of clearance at this point will prevent rubbing even if you hit a decent bump at a moderate speed while turning. I also know if you hit that same bump with 1/2” of clearance you will rub—been there, done that. Here is my often posted pic of my truck when I was running 2” of torsion bar crank, an aftermarket bumper, had tied back the parking brake cable, and had tied back and reformed the inner fender liners. Tires were General 285/75s on 16x8 wheels with 4.5” of backspacing. The steering wheel is turned about 3/4 travel. This is what you want to look for when checking tire clearance. You also need to check the clearance on the bumper with the tire turned about the same amount the other direction:
Realize that if you move back a small amount of fender well sheet metal at this point, you will gain a lot of clearance. Nar has a method that seems to look good and give a significant amount of extra clearance. Another option is to install Bushwacker cut-out fender flares. Bushwacker says you can run 35s with no lift with their fender flares.
Also, realize that if you crank the t-bars more than a couple inches the ride will become progressively worse. Cognito says their leveling kit—which includes upper a-arms and longer shocks—will allow 2-3” of t-bar crank while still providing a decent ride.
It is important to realize than when the suspension companies started making lift kits for the HDs, they recommended running no larger than 285/75R16s on 16x8 wheels with 4.5” of backspacing to ensure adequate tire clearance. Here is a picture of what a 4” Rancho with 285/75R16s looks like:
How much rubbing is a big deal? This is another fuzzy area. For guys like me, any rubbing is unacceptable, and I WILL notice it if my tires rub. However, when I ran the 285s with cranked t-bars I didn’t have any damage from the minimal rubbing though bumpy turns. Then again, during this period short period of time I didn’t have the truck out hunting either. If I had run chains on the front there would have been substantial rubbing, and I imagine chains rubbing that plastic inner fender liner would tear it up.
When running 285/75R16s, the bottom line is this. You WILL need to have some combination of cranked t-bars, modded inner fender liners, trimmed bumper, and get a front end alignment just to run these tries. If you have only a half-inch of clearance between the tire and the fender well when the tire is at the point of least clearance, you will most likely get rubbing at times, whether you notice it or not. It is harder to get tire clearance with the common 8” wheels than with the stock 6.5” wheels, but the 8” wheels will be more stable.
295/75R16 (33.42” x 11.61”)
I ran these on two different 4” lifts--set at 4” in the front, with plenty of clearance whether slaloming the truck on pavement or driving on rough hunting roads. I have no pictures of the tire clearance because it wasn’t remotely an issue. Here is a side view of the truck running a 4” Tuff Country lift with BFG AT 295/75R16s on my 16x8s w/4.5” of backspacing:
I know one guy who ran these tires on stock wheels with 2” cranked t-bars, all the fender liner mods, trimmed bumper, and didn’t have any complaints. He towed heavy and drove a lot on rough terrain. I’d bet money he had rub marks on his inner fender liners. I think 295/75R16s on 16x8s w/4.5” BS could be run with no risk of rubbing with the Cognito leveling kit set at 3” of lift. I am unsure of the ride quality with t-bars that high, but Cognito says it will be fine and they do seem to know what they are doing.
315/75R16 (34.60” x 12.40”)
The simplest way to run this size of tire is with a 6” lift. When HD lifts first hit the market, the suspension companies recommended this tire be run with a 6” lift. Both Rancho and Tuff Country—not sure of the others—are now saying you can run up to 35s with their 4” lift kits if you do the fender well mods. You will probably also need to crank the t-bars a little. Here are pics of my truck’s current configuration running BFG AT 315/75R16s on my 16x8s with 4.5” BS. I have a Tuff Country 4” lift set to 5.18” in the front (1 1/8” of t-bar crank), and I have 3.5” of lift in the rear. My parking brake cable is tired back and my inner fender liners have been reformed. I have about an inch of clearance between the tire and the fender well with the steering wheel turned 3/4 turn. My truck is very stable and I haven’t had any rubbing.
Bigger
I do not have any personal experience running tires larger than 315s. However, it seems that 37s will fit with a 6” lift with fender well mods and cranked t-bars.
Miscellaneous
Cranking torsion bars:
Two inches seems to be about the max you can crank the torsion bars without affecting ride quality, regardless of what torsion bar keys you use. As you go above two inches, the ride stats to become progressively more jouncy. Some guys are fine with the ride with their ride quality with the t-bars at 2.5”, others are not. Sometimes stock torsion bar adjusters max out at just under two inches of additional lift. There doesn’t seem to be any huge issues with running the HD CVs at the increased angles that 2” of t-bar crank yields, nor does there seem to be any huge issues with the tie-rod angles. Longer shocks are just almost a “must have” when you crank your t-bars around two inches. Again, the Cognito leveling kit is supposed to allow up to 3” of t-bar crank without a reduction in ride quality. This kit comes with new upper a-arms, poly upper a-arm bushings, and they insist you run longer shocks. You also must get an alignment after cranking the t-bars because cranking the t-bars changes both toe and camber.
This two inches of crank applies to both stock and lifted trucks. Most lift kits are designed around having the t-bars at stock angles when the front of the truck is set at the advertised height. You can normally go another two inches over that, though you might need aftermarket or the “Green” torsion bar keys to do that. The 4” Tuff Country is an exception, because when it is set for 4” of lift, the CVs are at less than stock angles.
Too much backspacing:
In all this discussion I have emphasized how you lose wheel well clearance when you reduce backspacing form the stock amount of 4.875”. However, if you run too much back spacing the tire and/or will rub on the steering knuckle, which is not good. For a while, Tophog ran a 16x8 with 5.25” backspacing and BFG AT 295s with a 4” Rancho lift. He said there was maybe 1/8” clearance between the knuckle and the tire. In looking at the clearance between the knuckle and my 315s, my guess is 4.75 to 5” is the max backspacing you can run with a 315 and a Tuff Country steering knuckle.
Pittman/idler braces:
Our steering linkage isn’t so stout—thank you GM engineers. The Pittman and idler will wear and get play in a relatively short time even when running stock tires, and larger tires just make it worse. The Cognito Pittman and idler braces are a good fix for this. Cognito tells you how to determine if your Pittman and idler are worn, and even tell you about how many miles you can run with what size tire before they start to wear excessively. I recently installed a new Pittman and idler along with the Cognito braces and the difference in steering response and stability was dramatic. I highly recommend these braces for any truck, especially if you are running larger tires.
Nar’s mods.
Here is the link to the thread where he describes how he did it. If enough of us keep bugging him, perhaps he’ll post pics.... ......
http://www.dieselplace.com/forum/showpost.php?p=644230&postcount=5
Bushwacker Cut-Out Fender Flares:
Bushwacker says you can run up to a 35” tire with these fender flares. You must cut through your fenders, starting 1.5” to the aft of the lower portion of the fender and angling all the way up to the top of the fender well. I strongly suspect you’d also need to crank the t-bars to run 35s, but that is just my guess.
Increase in front track.
From the factory, the front track of our 4x4 HDs is 2.5” wider than the rear track. The most durable lift kit are knuckle kits. (I will say there is one non-knuckle hit—the CST--that seems to be very durable.) All the knuckle kits currently available increase front track width over the stock amount. The least increase seems to come from the Tuff Country 4 and 6” kits and they add 3/8” per side. The new Cognito 4” kit is supposed to add only 1/2” per side. The other knuckle kits (like Rancho, Fabtech, Trailmaster) add 1.5” per side. The more a kit increases the front track, the less clearance you have in the wheel well, just as with wheel back spacing.
Determining tire size:
Here is how to break down a metric tire size. Let’s take a 315/75R16. “315” is the width of the tire in millimeters, sidewall to sidewall. To convert to inches we divide 315mm by 25.4 and get 12.4 inches. “75” is the ratio between the width of the tire and the height of the sidewall. So we multiply 12.4 x .75 and get a sidewall height of 9.3 inches. To get the height of the tire, we add the wheel size of 16 inches to the sidewall height x 2, which yields a total height of 34.6 inches. Realize those are nominal measurements and will vary some between tires.
For a thorough discussion of backspacing and offset, see this thread by “snuffysmith”: http://www.dieselplace.com/forum/showthread.php?t=78248
I probably need to review and edit this post a little better, but I am out of time and I want to get this info out. I hope it prevents guys from being surprised at the issues that come up when they run larger size tires/wheels.
Disclaimers/Philosophy
Opinions on what it takes to run a larger tire/wheel vary. These opinions vary based on a driver’s beliefs, perceptiveness, experience, conditions he/she drives in, and his/her driving style. Additionally, there are some minor differences in individual trucks that have a small play as well. Before someone’s opinion can be evaluated, you need to know about their opinions, experience, driving conditions, and driving style. Common statements like “I have run 475/30/27s for 100,000 miles with no lift and have had absolutely no problem” are of limited usefulness unless you know how the truck is being used and what the driver considers acceptable.
I am not the final word on any of this. I am not a professional mechanic. I am a 46 yr old "hobbyist" mechanic with advanced do-it-yourself skills. I tore into my first vehicle 29 years ago and have maintained, repaired, upgraded, and rebuilt several diverse types of vehicles—from drag race cars to daily drivers to 4x4s—since then. I have done this work in the garages of the many houses I have lived in using hand tools, floor jacks, an ever increasing inventory of specialty tools, and basic air tools. I hope the perspective of a “do-it-your-selfer” is useful to fellow do-it-your-selfers. No doubt the professional mechanics have the highest level of skill and knowledge, but it’s probably been a while since many of them had to roll around on the garage floor under a vehicle with a center section or transmission on their chest…………….
I am very perceptive (fussy?) concerning the operation of my truck. I have been a USAF pilot for almost 23 years, and it has become part of my nature to pay close attention what is going on with any vehicle I am operating. It is rare that a noise, vibration, gauge reading, or “feel” escapes me. I think many of us are that way, but I also realize many are not. Any tire rubbing at all is unacceptable to me, and I am not likely to miss it when a wheel does rub. That is an important point to keep in mind when I talk about what I think works and what doesn’t. I drive fairly aggressively, tending to “launch” the truck from stops and tending to “slalom” when I turn corners, though I usually drive very close to the speed limit. However, here in AZ, that means 75-80 mph every day I drive to work. I bought my truck to be a 20 yr truck that could do anything and everything in a very stable and controlled manner, which includes: hauling the family, hunting on rough roads and mildly off road, hauling heavy, towing heavy, and taking me to work every day. I didn’t realize I would be slowly sucked into running more and more power…………
My Truck
I have a 2002 Chevy, crew cab, short bed, 2500 HD DMax/Alli. I bought it new in June of 2002 and currently have 60,000 miles on it. So far, the truck has taken me to work, hauled the family a fair amount, taken me hunting in rough roads/off road, hauled heavy a few times, and towed medium a couple times. I have not yet towed heavy. I lived near San Antonio the first 2.5 years I had the truck and have lived near Tucson for the last 1.5 years. I am driving five times as far to work now than I was when I lived in Texas.
On this truck I progressed from: running 265s on stock rims, to cranking t-bars w/aftermarket keys and adding Bilstein shocks, to doing all the fender mods and running the 285s on 16x8s (4.5" BS) that sometimes rubbed, to installing (myself) a 4" Rancho (with 3.5" rear lift) and running 295s, to swapping the Rancho front lift for a Tuff Country 4" front lift, to finally going to 315s which require 5 1/8" of lift in the front. This doesn't count the three sets of t-bar relocators, the three sets of t-bar keys, or the 1/2 dozen stitches I needed when a t-bar adjusted bolt stripped and launched the breaker bar into my fore head.
Issues with Larger Tires and Wheels
Please note. I am using 16” wheel size as my basis, but the same issues apply to other sizes as well. The critical factors are the tire height, tire width, and wheel backspacing.
Stock 245/75R16 (30.47” x 9.65”). The factory combination for 2500s is LT 245/75R16s on 16x6.5” wheels with 4.875” of measured backspacing. (Please note some may call that 5” of backspacing.) Some dealers are selling trucks with 265/75R16s on the factory 16x6.5” wheels—my friend just bought one so equipped. Several have posted that the 3500 SRW comes with a 16x7” wheel.
265/75R16s (31.65” x 10.43”)
I ran 265/75R16s on a bone-stock suspension with no tire clearance problems. Many others here have reported the same results. I took the truck hunting in this configuration, crammed the skid plates full of mud and vegetation, so I added aftermarket torsion bar keys, cranked the t-bars 1.75”, and added stock height Bilstein shocks with 1” spacers under the top mount. I did not need to get the aftermarket torsion bar keys to get 1.75” of lift. After I cranked the t-bars I went hunting over the same terrain and had no ground clearance issues.
265/75R16s seem to work very well on the factory wheels, even though the stock rims are 1/2” narrower than what the tire manufacturers recommend for 265/75R16s. You shouldn’t have to crank the t-bars or do any other mods to run 265/75R16s on stock wheels without rubbing.
I don’t know if you’d have to crank the t-bars to run aftermarket wheels with the 265s, assuming you are running a typical 16x8” wheel with around 4.5” of backspacing. Many guys crank the t-bars to level the truck when they install 265s on aftermarket wheels because they like the leveled look, but that doesn’t mean the t-bars must be cranked.
285/75R16 (32.83” x 11.22”)
Here is where the issues and controversies seem to be the worst. You have guys like me who insist this tire will rub without a lift and you have guys who insist they have had absolutely no rubbing of any kind. Let’s start with there is agreement. To run a 285/75R16 or similar size tire you will have to crank the torsion bars and/or work on the inner fender liners and trim the air dam of the front bumper. Everyone seems to at least do those things when running this size of tire, and that is where the agreement ends.
You will get better clearance running the 285s on stock wheels. This is because the stock wheel is narrower and has more backspacing than the typically used aftermarket wheels. The stock wheel is 6.5” wide and has a 4.875” of backspacing. This means the center line of the wheel and tire is 1.625” toward the inside of the wheel well. An 8” wide wheel with 4.5” of backspacing has its centerline .5” toward the inside of the wheel well. This extra 1.125” the 8” wheel is moved toward the outside of the fender well causes a reduction of tire clearance when turning. It causes a reduction in clearance because the tire is on a longer moment arm and moves in a larger arc, meaning it comes closer to the wheel well during turns. Again, you’ll get the better clearance running 285/75R16s on the stock wheels than you will wider wheels with less backspacing.
However, there is a potential problem running 285/75R16s on the stock 6.5” wide wheels. The tire manufacturers recommend a 7.5” minimum rim width for a 285/75R16. Why? On too narrow a rim, the tire tread tries to curve across the width vs remaining flat. If you run a tire with the tread rounded across its width, you will wear out the centers—been there, done that. In order to get the width of the tread flat you have to run less PSI. When you run less PSI the sidewalls are less stiff, meaning the tire will flex more, meaning you have less stability.
How much less stability? That is the fuzzy area. You have guys like me to whom the idea of even the potential of less stability is heinous. After all, I can’t slalom my truck around turns if the sidewalls flex too much. Further, hauling a heavy load or a heavy trailer—especially on a windy, twisty road—with sidewalls that flex too much would not be pleasant. However, if you never tow/haul heavy or slalom, then the stability of a 285 on the stock wheel probably isn’t something you’d even notice. Others here have talked about towing light to medium loads without a significant loss in stability, probably because the good all terrain tires are designed to be “aired down” to improve traction in certain off road situations. If they have strong enough sidewalls to run aired down, then chances are they will maintain decentstability at lower air pressures with moderate loads. Regardless of all that, running a 285 on a 6.5” rim will not be as stable as running a 285 on an 8” rim, and not every tire company will install tires on wheels that are that much narrower than what the tire manufacturer recommends.
No matter which wheel you run with your 285/75R16, the tight point of tire clearance is between the rear outside of the tire and the rear outside of the fender well when the steering wheel is turned about 3/4 travel. I do not have a hard and fast rule, but with the various tire sizes and lifts I have run, it seems that about 1” of clearance at this point will prevent rubbing even if you hit a decent bump at a moderate speed while turning. I also know if you hit that same bump with 1/2” of clearance you will rub—been there, done that. Here is my often posted pic of my truck when I was running 2” of torsion bar crank, an aftermarket bumper, had tied back the parking brake cable, and had tied back and reformed the inner fender liners. Tires were General 285/75s on 16x8 wheels with 4.5” of backspacing. The steering wheel is turned about 3/4 travel. This is what you want to look for when checking tire clearance. You also need to check the clearance on the bumper with the tire turned about the same amount the other direction:
Realize that if you move back a small amount of fender well sheet metal at this point, you will gain a lot of clearance. Nar has a method that seems to look good and give a significant amount of extra clearance. Another option is to install Bushwacker cut-out fender flares. Bushwacker says you can run 35s with no lift with their fender flares.
Also, realize that if you crank the t-bars more than a couple inches the ride will become progressively worse. Cognito says their leveling kit—which includes upper a-arms and longer shocks—will allow 2-3” of t-bar crank while still providing a decent ride.
It is important to realize than when the suspension companies started making lift kits for the HDs, they recommended running no larger than 285/75R16s on 16x8 wheels with 4.5” of backspacing to ensure adequate tire clearance. Here is a picture of what a 4” Rancho with 285/75R16s looks like:
How much rubbing is a big deal? This is another fuzzy area. For guys like me, any rubbing is unacceptable, and I WILL notice it if my tires rub. However, when I ran the 285s with cranked t-bars I didn’t have any damage from the minimal rubbing though bumpy turns. Then again, during this period short period of time I didn’t have the truck out hunting either. If I had run chains on the front there would have been substantial rubbing, and I imagine chains rubbing that plastic inner fender liner would tear it up.
When running 285/75R16s, the bottom line is this. You WILL need to have some combination of cranked t-bars, modded inner fender liners, trimmed bumper, and get a front end alignment just to run these tries. If you have only a half-inch of clearance between the tire and the fender well when the tire is at the point of least clearance, you will most likely get rubbing at times, whether you notice it or not. It is harder to get tire clearance with the common 8” wheels than with the stock 6.5” wheels, but the 8” wheels will be more stable.
295/75R16 (33.42” x 11.61”)
I ran these on two different 4” lifts--set at 4” in the front, with plenty of clearance whether slaloming the truck on pavement or driving on rough hunting roads. I have no pictures of the tire clearance because it wasn’t remotely an issue. Here is a side view of the truck running a 4” Tuff Country lift with BFG AT 295/75R16s on my 16x8s w/4.5” of backspacing:
I know one guy who ran these tires on stock wheels with 2” cranked t-bars, all the fender liner mods, trimmed bumper, and didn’t have any complaints. He towed heavy and drove a lot on rough terrain. I’d bet money he had rub marks on his inner fender liners. I think 295/75R16s on 16x8s w/4.5” BS could be run with no risk of rubbing with the Cognito leveling kit set at 3” of lift. I am unsure of the ride quality with t-bars that high, but Cognito says it will be fine and they do seem to know what they are doing.
315/75R16 (34.60” x 12.40”)
The simplest way to run this size of tire is with a 6” lift. When HD lifts first hit the market, the suspension companies recommended this tire be run with a 6” lift. Both Rancho and Tuff Country—not sure of the others—are now saying you can run up to 35s with their 4” lift kits if you do the fender well mods. You will probably also need to crank the t-bars a little. Here are pics of my truck’s current configuration running BFG AT 315/75R16s on my 16x8s with 4.5” BS. I have a Tuff Country 4” lift set to 5.18” in the front (1 1/8” of t-bar crank), and I have 3.5” of lift in the rear. My parking brake cable is tired back and my inner fender liners have been reformed. I have about an inch of clearance between the tire and the fender well with the steering wheel turned 3/4 turn. My truck is very stable and I haven’t had any rubbing.
Bigger
I do not have any personal experience running tires larger than 315s. However, it seems that 37s will fit with a 6” lift with fender well mods and cranked t-bars.
Miscellaneous
Cranking torsion bars:
Two inches seems to be about the max you can crank the torsion bars without affecting ride quality, regardless of what torsion bar keys you use. As you go above two inches, the ride stats to become progressively more jouncy. Some guys are fine with the ride with their ride quality with the t-bars at 2.5”, others are not. Sometimes stock torsion bar adjusters max out at just under two inches of additional lift. There doesn’t seem to be any huge issues with running the HD CVs at the increased angles that 2” of t-bar crank yields, nor does there seem to be any huge issues with the tie-rod angles. Longer shocks are just almost a “must have” when you crank your t-bars around two inches. Again, the Cognito leveling kit is supposed to allow up to 3” of t-bar crank without a reduction in ride quality. This kit comes with new upper a-arms, poly upper a-arm bushings, and they insist you run longer shocks. You also must get an alignment after cranking the t-bars because cranking the t-bars changes both toe and camber.
This two inches of crank applies to both stock and lifted trucks. Most lift kits are designed around having the t-bars at stock angles when the front of the truck is set at the advertised height. You can normally go another two inches over that, though you might need aftermarket or the “Green” torsion bar keys to do that. The 4” Tuff Country is an exception, because when it is set for 4” of lift, the CVs are at less than stock angles.
Too much backspacing:
In all this discussion I have emphasized how you lose wheel well clearance when you reduce backspacing form the stock amount of 4.875”. However, if you run too much back spacing the tire and/or will rub on the steering knuckle, which is not good. For a while, Tophog ran a 16x8 with 5.25” backspacing and BFG AT 295s with a 4” Rancho lift. He said there was maybe 1/8” clearance between the knuckle and the tire. In looking at the clearance between the knuckle and my 315s, my guess is 4.75 to 5” is the max backspacing you can run with a 315 and a Tuff Country steering knuckle.
Pittman/idler braces:
Our steering linkage isn’t so stout—thank you GM engineers. The Pittman and idler will wear and get play in a relatively short time even when running stock tires, and larger tires just make it worse. The Cognito Pittman and idler braces are a good fix for this. Cognito tells you how to determine if your Pittman and idler are worn, and even tell you about how many miles you can run with what size tire before they start to wear excessively. I recently installed a new Pittman and idler along with the Cognito braces and the difference in steering response and stability was dramatic. I highly recommend these braces for any truck, especially if you are running larger tires.
Nar’s mods.
Here is the link to the thread where he describes how he did it. If enough of us keep bugging him, perhaps he’ll post pics.... ......
http://www.dieselplace.com/forum/showpost.php?p=644230&postcount=5
Bushwacker Cut-Out Fender Flares:
Bushwacker says you can run up to a 35” tire with these fender flares. You must cut through your fenders, starting 1.5” to the aft of the lower portion of the fender and angling all the way up to the top of the fender well. I strongly suspect you’d also need to crank the t-bars to run 35s, but that is just my guess.
Increase in front track.
From the factory, the front track of our 4x4 HDs is 2.5” wider than the rear track. The most durable lift kit are knuckle kits. (I will say there is one non-knuckle hit—the CST--that seems to be very durable.) All the knuckle kits currently available increase front track width over the stock amount. The least increase seems to come from the Tuff Country 4 and 6” kits and they add 3/8” per side. The new Cognito 4” kit is supposed to add only 1/2” per side. The other knuckle kits (like Rancho, Fabtech, Trailmaster) add 1.5” per side. The more a kit increases the front track, the less clearance you have in the wheel well, just as with wheel back spacing.
Determining tire size:
Here is how to break down a metric tire size. Let’s take a 315/75R16. “315” is the width of the tire in millimeters, sidewall to sidewall. To convert to inches we divide 315mm by 25.4 and get 12.4 inches. “75” is the ratio between the width of the tire and the height of the sidewall. So we multiply 12.4 x .75 and get a sidewall height of 9.3 inches. To get the height of the tire, we add the wheel size of 16 inches to the sidewall height x 2, which yields a total height of 34.6 inches. Realize those are nominal measurements and will vary some between tires.
For a thorough discussion of backspacing and offset, see this thread by “snuffysmith”: http://www.dieselplace.com/forum/showthread.php?t=78248
I probably need to review and edit this post a little better, but I am out of time and I want to get this info out. I hope it prevents guys from being surprised at the issues that come up when they run larger size tires/wheels.