Did you run out of RPM? Did you find it necessary to re-gear even with all your performance goodies?

I almost bought my set of gears then heard from someone based on his experience that it wasn't needed. In fact, this person went with a high ratio of 3.55 with 40" tires:eek: I don't know what route to take now:confused: Any help? Thanks. Nick

If you put on larger tires and regear, you should get back close to the stock ratio, so you should not run out of RPM. Having larger tires and not regearing, you loose power and will eventually run out of RPM at the higher speeds due to higher rotations needed to get a a full revolution of the tire.

That doesnt sound right at all, going a higher gear for a larger tire? I think you'd be capped at driving like 70mph or something.

Regearing is a proper way to do things if you are getting tires larger than 33" and you would definately want to put some of that power back to the floor after adding more goodies!

4.56's might be a bit steep. I was thinking 4.10's to go with my 315's (H2's) might get me back close to normal......I know we discussed this before, but never came up with a definite answer on gearing.

Tom :pig:

I think it is the other way around cid', I have 37"s with the stock 3.73's and my rpms stay lower than stock since a bigger tire is making slower revolutions, If you have small tires then they will be spinning faster. I have gone like 120 in my truck according to the Juice and not ran out of rpms. Yes you do loose power but with the duramax it isn't as bad as having a gasser. I just ran a 15.5 at LACR which has a crappy altitude, on regular Juice/Attitude, level four only not five since I still have a stock tranny.
Jason

I'd regear if u tow alot and pull max weights, but just playing w/ the truck I'd leave it stock gear.

BTW if u are going for low 12's quarter mile u don't want the lift and big tires anyway and as Pat found out the dmax didn't like a low gear ratio for racing.

I rarely tow and when I do it's a couple of lawn mowers. My goal really is the 12's but seeing as I already spent a good chunk of change on the lift, tires, wheels etc. I want to leave them in place.

What was wrong with the lower gears that Pat was running? I must've missed that thread. Thanks. Nick.

It wasn't actually gears, but I changed the effective gear ratio by putting smaller tires on.

The truck did not go any quicker, but should have just due to the 200lb weight reduction. Nope.

I think it is the other way around cid', I have 37"s with the stock 3.73's and my rpms stay lower than stock since a bigger tire is making slower revolutions, If you have small tires then they will be spinning faster. I have gone like 120 in my truck according to the Juice and not ran out of rpms. Yes you do loose power but with the duramax it isn't as bad as having a gasser. I just ran a 15.5 at LACR which has a crappy altitude, on regular Juice/Attitude, level four only not five since I still have a stock tranny.
Jason ditto...........at some point your going to have to decide on whether you want a 12 second truck or a quick truck with big tires............the limits of your axles, tranny and driveshafts will be found trying to launch 36's.

I run 34" tires and 19.5 wheels with 4.10 rear end gears with a dually. With this set up I can run 8400lbs down the track all day long and get 14.3's. With my set up the 4.10 are helping launch the 8400lbs. John

Jason,

Thanks for pointing it out, makes more sense after looking back at what I posted now.

anyone know what gears sp33d runs on his lb7? he would be a help.

You don't need to regear unless you're planning on towing.. I know it sounds crazy for me to say this, though my truck did great with 3.73's and 39 inch mickey thompson baja's.. It ran it's best (at that time) with the larger tires.. I went from a 6 inch and 35's to a 9 inch and 39's.. It ran 14.1 in 2wd with the 39's (actually lowered my times a couple tenths from the 35 buckshots in 2wd).. I never launched in 4wd with it lifted.. Not long after this, I tore everything out of the front end at a truck pull.. I took the suspension lift off and it now runs the same 14.1 in 2wd with 35's.. You can also lockout overdrive with the large tires and it has just enough rpm to run the quarter in 4th gear.. The fuel mileage remains good as well.. Mine remained to be 14 to 16 in town, and better on the highway.. Good luck...

You don't need to regear unless you're planning on towing.. I know it sounds crazy for me to say this, though my truck did great with 3.73's and 39 inch mickey thompson baja's.. It ran it's best (at that time) with the larger tires.. I went from a 6 inch and 35's to a 9 inch and 39's.. It ran 14.1 in 2wd with the 39's (actually lowered my times a couple tenths from the 35 buckshots in 2wd).. I never launched in 4wd with it lifted.. Not long after this, I tore everything out of the front end at a truck pull.. I took the suspension lift off and it now runs the same 14.1 in 2wd with 35's.. You can also lockout overdrive with the large tires and it has just enough rpm to run the quarter in 4th gear.. The fuel mileage remains good as well.. Mine remained to be 14 to 16 in town, and better on the highway.. Good luck...not that im doubting you, those times just seem so unbelievable.

Thanks for the replies guys. Morse, I see what you mean. Did you notice a difference with the bigger meats when you shot out of the hole? I know that Clint's truck over at ATS does 12's with his Dmax. And his has 40" tires with a gear ratio of 3.55. Check it out on their web site or hese at http://www.atsdieselperformance.com/aboutus/Videos_A1.asp

The large meats narmally didn't hook up very well.. They did great prep work on the track, and I was able to cut a 1.9 60' with the baja belted tires (in 2wd)..
Dmax4u2nv, I can see where some would seem it far fetched for a truck to actually speed up going to a larger tire.. I was pleasently surprised myself.. I raced all year long in the DHRA here in the southeast on 35's and managed to be consistant enough to win the bracket division for the year.. My dial in was alway's in the high 13's to low 14's depending on how I decided to set my programs.. An attitude alone can easily get a completely stock truck down in the 14's..

Thanks for the replies guys. Morse, I see what you mean. Did you notice a difference with the bigger meats when you shot out of the hole? I know that Clint's truck over at ATS does 12's with his Dmax. And his has 40" tires with a gear ratio of 3.55. Check it out on their web site or hese at http://www.atsdieselperformance.com/aboutus/Videos_A1.asp

I'm pretty sure Clint at ATS is runing 35" tires on his truck with a 6" lift

Hmmm. Thought he was running 40's? My mistake, sorry. Nick

I ran the stock 3.73 gears in my LB7. The largest tire I ran was a LT325 (35.2"). I never have had small tires on the truck at a drag strip so I don't know what it does or doesn't do for you. I hear/read/talk to some that say it helps. Others no. Some say regearing is good for larger tires. Others no.

I'm stuck right now trying to decide between lift and 35" tires (looks) or level it out and run something a bit smaller (speed, in theory) for my new truck. If the bigger tires do slow you down I think it's only due to the extra weight they add but at some point "science" has to take over I would think. I'm sure the lift and large tires have to be slowing me down at least a couple tenths of a second, but I honestly don't know how many (if any).

I had the same dilemna as Sp33d last year regarding the nice looks of a 6" lift w/35s vs. stock height with larger tires. I ended up lifting my truck 4" and going with 33's as a compromise. To me it looks better than stock though not as nice as some 6" lifts. It behaves well at the track and the modest lift doesn't seem to hurt it much in sledpulling. Towing my 10k bumper pull trailer last year wasn't a problem with the 33's. What's nice about the smaller lift is the bedrails are still low enough to pull most 5th wheel trailers.

Doug

I'd like to echo Morse's comments as well. My '72 K5 Blazer with a supercharged 454 ran .5 seconds guicker with 40" Swampers vs. 35" BFG Mud TAs. Same scenario with my diesel. These big blocks/diesels like to utilize the tq and pull, pull, and pull!

I know that having a 373 gear and larger tires looses power, but what kind of power are we talking about? TQ or ability to reach a top speed drag?

Hey I Had 4o" Tires With 4.63 Gears And I Loved It But With Gears You Will Trip The Sevice Brake System And Your Abs And Brake Light Will Come On

http://www.richmondgear.com/01calculators.html

cid ever notice the difference between Hi range and LOW range? its not as drasic of a difference but you will lose pulling power, acceleration and possibly milage by moving to a higher gear ratio(lower number) The difference may or may not be noticable depending on your specific combination. A 373 to a 411 isnt much of a change....you will notice it but its not huge. There are many people that gain MPG buy going to deeper or lower gear(higher number) when running big tires since the motor isnt working as hard to maintain and achieve speed even though the RPM's are brought down.

IMO decide what ever tires you want to run and get a close to stock ratio. Lean a bit to the lower or the higher depending on what you are doing with it and what you may do to it(change tire size agin for instance)

Ok, I did a few calculations and came up with this: My stock tire height was 30.0" and my present tires are 36.8" So 30.0 divided by 36.8 multiply by 3.73 gives me an effective ratio of 3.04. Damn, that is wayyyy to high!!

Now lets try this: The tires on now are 36.8" divided by the original size of 30.0" multiply by 3.73 would tell me that I need a ratio of 4.56. Wow, I did that myself??? And my math teacher failed me in math. Lol.