I currently have 4.10 gears in my suburban. With the rising(quickly) price of diesel I would like to change the gears. The question is, how will this affect my mileage and my towing capablilty? I currently get 12/13 in town and 15 on the freeway, but I have to push 2800 rpm to maintain freeway speeds and have a lot of people passing me. I tow about 8,000lb total during the summer and usually only on back roads and mountains. Should I go to a 3.73? And how hard is it to do yur self? What gear combinations are you using and what is your milage? My towing milage is between 9 and 11.

Thanks,

what size tires are you running? WHat are "freeway speeds" in your area? Those RPM's sounds abit high, hopefully some of these auto guys will pipe up shortly

I am running 285/75/16 BFGoodrich. And Freeway speeds are 75 mph.

Your freeway speeds vs. rpm sound about right to me. I hate driving my burb on the interstate (~80mph) cuz I don't like getting passed and I don't like running the heck out of it at 2700 rpm.

But you better run the math first. I think it'll take an awfully long time and a lot of miles for the cost of the gear change to be realized in increased fuel economy.

I know if I ever shop for another 6.5 powered vehicle in the future, I'll be looking for 3.73's.

I have the 3.73 gears and to tell you the truth I'm not getting anything better. I get 14 mpg with around the town milage. I also have a cat on mine which doesn't help and a stock exhaust. For what your going to be giving up it's not worth it. I tow around the same weight in the summer and I need to drop down to 40 mph going up any long step grades or my temp goes way up. Once I get a new exhaust on the system I hope my mpg goes up some.

changed from 4.10 to 3.73, Lost MPG with a loaded truck camper,
TCC locks up by Transmission speed not Speedo input,
Use to lock up at 45 mph, now with 3.73 locks up at 50 mph,
that and " more throttle required " makes the TCC unlock sooner and downshift sooner.
I would not do it again,
MPG empty , Slight increase, highway.....

try some bigger tires... 1 iinch Dia increase approx 3 percent decrease in MPH.

boisebiker,

try and find a speedometer/odometer recalibrator for your truck. you are running a tire that is 2 sizes bigger. you might find out that your mileage will increase if you recalibrate it since right now you are covering more ground then what the odometer is picking up. summitracing.com has a couple to pick from.

boisebiker,

try and find a speedometer/odometer recalibrator for your truck. you are running a tire that is 2 sizes bigger. you might find out that your mileage will increase if you recalibrate it since right now you are covering more ground then what the odometer is picking up. summitracing.com has a couple to pick from.Or all you have to do to get an accurate estimate of your mileage is multiply the odometer times the ratio of the circumference of the bigger tire over the circumference of the stock sized tire.

But it doesn't amount to much of an increase.

Example:
diameter of 285's (32.83)
diameter of the 265's (31.65)

320 miles on the odometer when you fill up. Say it took 22 gallons to fill, then
320/22 = 14.55 mpg

But correcting for the tire size...
320 x (32.83/31.65) / 22 = 15.09

So you're actually getting about a 0.5mpg better than you think you are.

I believe the speedo calibation is done thru the PCM or the computer.

Speedo cal is done thru the VSSB buffer, someone recently last week IIRC posted lengthy dissertation how it's done, sumtin' wrong with your Burbs mileage DG if all you ar getting is 14, maybe it's more weight Burb vs truck or your spedo is off, with my 3:73s on 265/75 AT tires I get 16-17 running 65-70mph, have hit 20 before running at 55 but that was agony my truck does not like to go that slow:D . My Dad was following and he doesn't like to go faster than posted speed limit, only time I get in the low-teens is towing where I get 12-13mph or going very fast 75+ or ++:D. where my truck is very happy to run at.

After the signal leaves the buffer, where does it go?

PCM reads what buffer sends to it, I'll see if I can the post it may have been on GM-Diesel site also

BT

In this post I think is info you seek http://www.dieselplace.com/forum/showthread.php?t=24991&page=3&pp=10&highlight=speed+buffer%23post503770

After the signal leaves the buffer, where does it go?The VSS buffer converts the low voltage AC signal from the speed sensor into signals that not only the PCM can read, but also out to the cruise control, ABS, power steering, transfer case module, and stereo (for speed control volume).

Speedo cal is done thru the VSSB buffer, someone recently last week IIRC posted lengthy dissertation how it's done, sumtin' wrong with your Burbs mileage DG if all you ar getting is 14, maybe it's more weight Burb vs truck or your spedo is off, with my 3:73s on 265/75 AT tires I get 16-17 running 65-70mph, have hit 20 before running at 55 but that was agony my truck does not like to go that slow:D . My Dad was following and he doesn't like to go faster than posted speed limit, only time I get in the low-teens is towing where I get 12-13mph or going very fast 75+ or ++:D. where my truck is very happy to run at.
TD when I tow it has drop to 10 MPG. I to run 265/75/16 coopers soon to be replace with same size tires. I have an F engine and do not have the EGR. After talking to a few people it seems it might be the CAT doing it to me. At 124K the CAT might be restricted. If you or anyone else has an idea of something else that might be robbin me of MPG then let me know.

I am a big numbers kind of guy and I always compare the actual miles traveled to the odometer so I know that the odo is not correct and I adjust my mileage accordingly. I talked to a local shop and they said $800 minimum to change the rear end. And then I have to do the front. I just think that I could do it myself for about $180, but need to decide if it is worth it.

TD when I tow it has drop to 10 MPG. I to run 265/75/16 coopers soon to be replace with same size tires. I have an F engine and do not have the EGR. After talking to a few people it seems it might be the CAT doing it to me. At 124K the CAT might be restricted. If you or anyone else has an idea of something else that might be robbin me of MPG then let me know.
My MPG's were slowly dropping off and I made a couple of changes that brought me back up. First off the phoney little vacum lin that runs the Waste gate broke, and was worthless no boost, I replaced the vacum waste gate controller with a TurboMaster and MPG went up some. I put in new injectors and MPG and power went up. I put a Heath Chip in and power went up MPG stayed about the same. My Kitty Cat disappeared and low and behold boost, power and MPG went up. I'm back to 16-17 mpg back and forth adn around town and towning 5K water trailer couple 3 times a week and run 19.5-20.5 at 75 empty.

A cat ectemy is prescription than needs filled, if visuals are required then a cat enema, with an interior replumb of of the main artery thru the shells center say 3.5" of new cat gut would be sufficient to fix that constipated feeing your vehicle is ailing from.

BB if you are adept at diff set up then $180 self do would not take long to pay for itself at current $2+/gal pricing, heck if trend continues might even be worth paying someone to do it 3:73 gears are plenty for towing unless frequent heavy tows might be worth it to stay with 4:10s. You will have to run the numbers vs your towing power needs.

A cat ectemy is prescription than needs filled, if visuals are required then a cat enema, with an interior replumb of of the main artery thru the shells center say 3.5" of new cat gut would be sufficient to fix that constipated feeing your vehicle is ailing from.
So there should be no worries in passing emissions with a cat enema? Anybody performed this operation in a state with diesel emissions?

Chad

So there should be no worries in passing emissions with a cat enema? Anybody performed this operation in a state with diesel emissions?

Chad
Chad, last year I had my 97 tested for the first time (moved here in '03) with stock programming and aftermarket 3" exhaust. Passed the test with flying colors (well within CO standards), except that I was missing the Cat. Prompted by the tester, $20.00 suddenly resolved the lack of a Cat.;)

PM me if you want the name and address of the shop (in Aurora). Real nice guy.

In Idaho we are only tested on smoke. But they still look for the cat, had a enema done but didn't notice any difference. Power and mileage are the same, same noise also. And at $2.45 and rising it doesn't take long to pay off any midification that will increase the mileage. Has any one changed gears, how hard is it to do? Also working on an intercooler, will this help mileage?

Going back a few years on this. I blew the gears in a truck and put 4:10's in. Not hard at all. I'm not sure but I seem to remember that the axle shafts had to be replaced too. They where a little thicker. Going back more than 15 years. If you get someone to put all the gears together it's even easier. ;)

I personally have not changed gears, well I haven't done the work at least. It definately isn't cheap to have done, I had the front axle done in my jeep to the tune of about $750. I would search some of the 4X4 sites like jeepsunlimited.com or coloradok5.com for information on the subject. You should find some write ups.

The only towing I do is my 1700lb fishing boat so that's why I searched for the k1500 sub that I bought, which comes with 3.42 gears. MPG is real reason I own my truck. http://www.dieselplace.com/forum/images/smilies/rockon.gif

Chad

I went from 3:73 to 3:42 I figured way back then when I did it it would take about 60K miles to pay for itself, I now have about another 90K miles on since payback. I tow regular I do tow up to 10K which is bit much. I have 2x4 so only on axel to change.
I did exhaust then about 5K later axel ratio I gained about 15% in MPG between the two.

I replaced my gears last summer. I've got 245/75/16's. I originally had 4.10 gears and was getting a consistant 14MPG. We switched to 3.42's and my milage is now at 18MPG. Paid just under $500 for the front and rear gear sets, shipped. Plan on a $40 bearing or two and some seals also. The rear was a piece of cake, but the front was another story. The front unit is HEAVY and is hard to twist and turn back into place. (Only dropped mine once...Man that was loud!!).

For speedometer control, I bought the Superlift TruSpeed. 4WheelParts will beat anyone's price. I think I paid about $170, shipped. Anyway, it allows you to adjust your speedometer to match your gear and tire setup. Mine is within a 1/2 MPH @ 65MPH (according to GPS). It's a little pricey, but I know that it's right and that I can adjust it when I change tires later this year.

I gave up towing capacity for economy. Most of the time, I only haul my kids around. When I made the change, my truck had 69,000 miles on it, so I figured I had plenty of time to make up the initial investment in fuel savings.

I hauled a 35ft gooseneck with a '72 Jeep Wagoneer loaded up and still did fine.

Hope this helped.

So there should be no worries in passing emissions with a cat enema? Anybody performed this operation in a state with diesel emissions?

Chad
Check out this post http://dieselplace.com/forum/showthread.php?t=25695
I just had mine done and it passed better than new.