Chevy & GMC Vans

What is the difference in teeth from a 3.24 and a 3.73? I did a quick search but no easy answer.

I bought and paid for mine to be swapped, but never actually counted teeth. Now my milage is so poor (13-14mpg) and the van still downshifts far too often. My wife complains it feels like the tranny is going to explode.
I'm going to count the teeth on the removed 3.24 to be sure it really is the removed R&P.



background: tall 16" tires and rebuilt tranny (see signature)

3.23.........42-13
3.73.........41-11
http://www.quadratec.com/jeep_knowledge ... le-137.htm

Awesome! Thank you sir.

it would make sense that when you put in lower gears, your fuel mileage would be less, and your engine will need to turn faster to get the same speed as before.
Sounds like your tranny may need attention if it isnt shifting as it should.
BTW, I am constantly wanting to shift my 5 speed from 5th to 6th, since I went from 3.08's to 3.73's

doyoulikeithere wrote:it would make sense that when you put in lower gears, your fuel mileage would be less, and your engine will need to turn faster to get the same speed as before.
Sounds like your tranny may need attention if it isnt shifting as it should.
BTW, I am constantly wanting to shift my 5 speed from 5th to 6th, since I went from 3.08's to 3.73's

Your story sounds like my story, I guess this is my reality. I just had the trans rebuilt just after the differential change because I thought something must be wrong. My new plan will be to go back to the 15" wheels with smaller tires to help make up the difference in gearing. I do not want to break open the diff a third time. Thanks for sharing, it shined some light on my situation.

I'm going to guess it's the tires more than the gears. As I understand it, MPG isn't solely a factor of engine RPM, but also engine vacuum; i.e. you may get better MPG if you are running 55 mph @ 2500 rpm @ 3/4 throttle than 55 mph @ 2000 rpm @ full throttle. Those numbers are fabricated to make a point. But that's how I understand it. But no matter what, your going to get worse mpg turning more mass (i.e. bigger/heavier tires)

As I have stated before, I keep very accurate gas logs and found that my MPG went down by at least 10% when I went from 30 9.50s to 31 10.50s.

I have also said it before that the tires are what probably made the most difference. What I would like to do is find a very strong but light weight wheel for my van and pick a tire out that is not real heavy yet has great road manners. I know for a fact this will improve mileage. Can you get a set of stock tires or slightly larger to test?

Here is an example: I did a test on my 2003 Ram 2500 4x4. It came stock with load range E LT265/70R17's on steel wheels. They weigh in at about 100# each. I average 10 to 10.5 mpg. I ran a set of passenger rated 265/70R17's on stock Dodge aluminum wheels that weighed in at 65# each for around 3,000 miles and my overall milage jumped to about 12 to 12.5. This was combined towing/empty driving. The problem with the light tire was that the truck felt squishy towing the camper.

Another suggestion would be too loose the roof mounted cargo box except when you really need it. I don't have personal experience with this, but have been told by others (on other cars) that it noticeably affects their mileage.

I agree, this seems to be the deciding factor. My plan is still to log 50k on these tires, then switch back to smaller 15", that's all I can do at this point. I'll never be able to sell these used for anywhere near what I have in them.

As for the roof box, it's only on when camping.

I am wondering why your trans is shifting so much? One of the reasons I wanted to go to lower gears was to reduce the amount of shifting in hilly terrain (especially down to 2nd gear) and it appears to have accomplished that.

Why is it that nobody ever thinks to reprogram the final drive ratio in the vehicle computer? What do you think tells your tranny to shift and when? You can't just swap tires and gears in a computer controlled vehicle and hope for the best performance.

I was of the understanding his final drive ratio (gears/tires) is matched to stock. If the speedo is correct and matches the rpm of the stock gearing, the ecu has no idea the tires or gearing have been altered. I think ?

He does have a Jet SCU-2000 so I would think he has re-programmed for the gears and taller tires. If I find the tranny shifting a lot, I drop into 3rd and run it that way until on flats or interstate running.

I have the Jet programmer as well and while I still have stock 3.73 gears, I have changed for tire diameter (31") speedo correction. I've played around a bit with transmission shift mph though it hasn't really been noticeable. I will try experiment with larger mph differences here soon.

Going from 29.2" Yokohama Geolandar A/T-S to 31" tires and RTT installed, I'm about 2mpg down on highway (14-15mpg) and around 12mpg city. Almost all of my highway driving with 31" tires has been fully loaded and RTT so I'd like to compare empty without RTT.

I usually end up dropping transmission into "3" on rolling and hilly terrain with the setup right now, can't wait to install a set of 4.10 gears and get back closer to stock final drive ratio. Probably going to sell stock 3.73 gears and G80 locker to recoup some cost when done.

Not all JET programmers are created equal. Some allow you to correct the speedo and adjust shift points for predetermined situations like towing, others like my JET DST allow you to alter and reprogram just about every LUT (look up table) in the system. There is a calculator built in that asks for tire size and gear sizes and then uses that data to create a new LUT for shifting. etc. Once it's set it uses that data always, not on the fly like the programmers that have buttons for street, tow, etc.