what gear ratio do i need?

[mike]

i just picked up an 87 comanche, 2wd, 4.0L 5-speed dana 35 rear axle. someone has either replaced the gears or the whole axle, because going down the highway at 55 in 5th the engine is running around 2,300 and the speedo is almost 10 mph off( i know this because my wife was behind me and flashed the lights when she hit 55 in her car and i was showing 45). this will be a daighly driver, 99% highway ,and i want to replace the gears or complete axle and i want the rpms down around 1,500 at 55. what ratio do i need for that? also it has stock size tires i think are 225/75-15. or if someone has an axle thats geared like what i want and want to sell it or trade for some parts i have let me know.

[Pete M]

If all you wanted is a correct speedo, it's much easier than swapping axles or gears. See this site:

http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm

 

--Pete (ex-admin)

[Stumpy]

if he changes the speedo gear change the RPM's?

[Pete M]

Nope, but it would fix his speedometer. The stock gears for a 4.0L/stick are 3.07s. 3.07s suck in a major way. I would go with 3.55s. This site has some calculators on it.

http://www.4lo.com/4LoCalc.htm

 

Lower RPMS don't necessarily directly lead to better gas mileage since the engine now has to work harder to maintain the speed.

[Eagle]

i just picked up an 87 comanche, 2wd, 4.0L 5-speed dana 35 rear axle. someone has either replaced the gears or the whole axle, because going down the highway at 55 in 5th the engine is running around 2,300 and the speedo is almost 10 mph off( i know this because my wife was behind me and flashed the lights when she hit 55 in her car and i was showing 45). this will be a daighly driver, 99% highway ,and i want to replace the gears or complete axle and i want the rpms down around 1,500 at 55. what ratio do i need for that? also it has stock size tires i think are 225/75-15. or if someone has an axle thats geared like what i want and want to sell it or trade for some parts i have let me know.

Send me a PM with your e-mail address, and I'll send you an Excel spreadsheet that will tell you more about tire sizes and gear ratios than you want to know.

 

The short answer is that, for stock tires, 3.54 is the optimum ratio. The 5-speeds came with 3.07s, but they are geared so tall that 5th gear isn't usable except on the highway, and even at 55 you're well below the torque peak of the engine. 3.54s are a better balanced choice.

[Oizarod115]

i just picked up an 87 comanche, 2wd, 4.0L 5-speed dana 35 rear axle. someone has either replaced the gears or the whole axle, because going down the highway at 55 in 5th the engine is running around 2,300 and the speedo is almost 10 mph off( i know this because my wife was behind me and flashed the lights when she hit 55 in her car and i was showing 45). this will be a daighly driver, 99% highway ,and i want to replace the gears or complete axle and i want the rpms down around 1,500 at 55. what ratio do i need for that? also it has stock size tires i think are 225/75-15. or if someone has an axle thats geared like what i want and want to sell it or trade for some parts i have let me know.

Send me a PM with your e-mail address, and I'll send you an Excel spreadsheet that will tell you more about tire sizes and gear ratios than you want to know.

 

The short answer is that, for stock tires, 3.54 is the optimum ratio. The 5-speeds came with 3.07s, but they are geared so tall that 5th gear isn't usable except on the highway, and even at 55 you're well below the torque peak of the engine. 3.54s are a better balanced choice.

 

oh yeah, i can't wait for 3.07s on 32"s :D

 

225s id say 3.55s are dead on, i drove a 5spd XJ with 3.73s and it was a little on the low side compared to my MJ with 3.07s which are a little high.

 

can see it now ":mad: is this kid gonna go? the lights green!" :popcorn:

[mike]

thanks guys, looks like i'll be looking for one in the 3 1/2 range. i'm going to take off the cover today and see whats in there, i bet it's in the mid 4's, the PO must have removed this axle from one that had some big tires and didn't think about the gears being different. :cheers:

[Oizarod115]

thanks guys, looks like i'll be looking for one in the 3 1/2 range. i'm going to take off the cover today and see whats in there, i bet it's in the mid 4's, the PO must have removed this axle from one that had some big tires and didn't think about the gears being different. :cheers:

well the 4cyl and 4.0liters have different ratios too, the 4cyl being a 4.10 in alot of cases, so that could be the problem