Matching Gear Ratios

[AndyA]

I've seen several posts on here that say 4WD cannot be used unless the gear ratios match front and back.

Looking at my window sticker for standard equipment it shows 3.07 axle ratio and then under the options it shows "[DME] 3.73 Axle Ratio". Is that just for the rear?

Looking at my build sheet it only says "3.73 Rear Axle Ratio" and doesn't mention the front.

I don't know yet what I actually have - I will jack it up and put some string on the driveshaft to measure, but I am confused about not using 4WD with a mismatch but that is how it came from the factory?

Thanks, Andy

 

[GPK03X]

You should be fine. Your Jeep is equipped with the optional 3.73 gears front and rear

 

[macleanflood]

I can't imagine a vehicle coming from the factory with mismatched gears.

Have you ever serviced the diffs? Changed the fluids?

Might be a good excuse to pull the covers and see what's stamped on the ring gears.

-Mac

 

[KingCarGuyZ]

There's no slip between the front and rear axle in the transfer case, so a difference in axle ratio on a high traction surface (like pavement or even grass) would cause binding in the transfer case and drivetrain components and likely break something quite quickly.

That being said no jeeps intentionally left the factory with differing axle gears, so if your jeep is still stock axles you are 99.9% gonna be just fine. this rule comes into play more when changing axle ratios to compensate for larger tires.

If you want to be 100% sure you can crawl under the jeep and read the gear ratios on the axle tags bolted to the pumpkin, but I am confident that you have both a front a rear 3.73 ratio and will be just fine. Jeep advertising just says 3.73 rear and doesn't mention the front.

 

[Jerry Bransford]

There's no slip between the front and rear axle in the transfer case, so a difference in axle ratio on a high traction surface (like pavement or even grass) would cause binding in the transfer case and drivetrain components and likely break something quite quickly.

It's unlikely to break anything when the ratios don't match perfectly as the drivetrain is stronger than most think it is. They will bind up on pavement even when the F/R ratios match perfectly like on a Rubicon and cause wonky steering on curves. And only Rubicons have perfectly matched F/R ratios. Front tires rotate more than the rears do during turns so binding occurs on high traction surfaces even with perfectly matched ratios like the Rubicon has.

And the F/R ratios don't perfectly match on non Rubicons, the front and rear axles of non-Rubicons have different diameter ring gears which causes all non-Rubicons to have a .01 ratio difference between the front and rear axles. 3.07/3.08, 3.73/3.75, 4.10/4.11, 4.55/4.56, etc. So if things were likely to break they'd be breaking all the time because way too many clueless owners commonly drive in 4Hi on the highway with non perfectly matched front and rear ratios. They don't know any better and often do it just because it feels a bit more "Jeep-like" with the quirky/jerky steering that happens when in 4x4 on pavement. There are high traction surfaces like some offroad trails like Moab have high traction surfaces, nearly as much traction as pavement provides and pretty much everyone on such trails stay in 4x4.

 

[AndyA]

OK, thanks makes sense! Andy

 

[OldBuzzard]

And the F/R ratios don't perfectly match on non Rubicons, the front and rear axles of non-Rubicons have different diameter ring gears which causes all non-Rubicons to have a .01 ratio difference between the front and rear axles. 3.07/3.08, 3.73/3.75, 4.10/4.11, 4.55/4.56, etc.

How does that work? A 4.56 Dana 30 has a 41-tooth ring and a 9-tooth pinion. A Dana 44 has the same tooth counts, just bigger.
41/9=4.55555 (and we round it up to 4.56, where some other car brands might round it down to 4.55).
Where do you get 4.55/4.56 on the TJ?

 

[rouxbicon]

I've seen several posts on here that say 4WD cannot be used unless the gear ratios match front and back.

Looking at my window sticker for standard equipment it shows 3.07 axle ratio and then under the options it shows "[DME] 3.73 Axle Ratio". Is that just for the rear?

Looking at my build sheet it only says "3.73 Rear Axle Ratio" and doesn't mention the front.

I don't know yet what I actually have - I will jack it up and put some string on the driveshaft to measure, but I am confused about not using 4WD with a mismatch but that is how it came from the factory?

Thanks, Andy

I'll bet what you're seeing on your window sticker is a list of all standard equipment for any base model TJ with the 4.0. Many individual items of the standard equipment list can be and often are superseded by various individual options and option packages. In your case the 3.07 axle ratio that was standard equipment for base model TJs with the 4.0 was superseded by the 3.73 ratio, probably as part of some option package.

Why they refer to it as a "rear" axle ratio for a 4wd vehicle rather than just an axle ratio is a mystery I can't anwer

 

[jjvw]

Put the Jeep in 4WD and the transmission in neutral and give it a push. If you can roll it around by hand without it binding up, then there isn't a mismatched gear ratio problem