Another Stupid Question on Gearing!

Okay, I've been reading all the threads on gearing and I just don't get it.... Maybe most people think automatic transmission, but my XJ is a standard and I'm not going to change that (although I may upgrade to an AX-15 at some point). Here's my stupid logic so someone can explain where I’m going wrong.

First of all, this will be a toy for running around town and to the beach with the dogs. No serious rock climbing or trail riding planned. If that changes, then I'll rethink the tire size and gear ratio thing.

My 89 4.0/BA-10 XJ came from the factory with 3.07 gears. Now, I'm going to assume that my XJ came with 28" tires and the engineers that picked the 3.07 factory gears knew what they were doing. I have a donor 89 XJ 4.0/AW4 with 3.55 gears. The plan was to put these axles on my XJ.

The way I figure the math, I can run 32's and still be very close to the original design gear ratio. According to the posts I've read here, 3.55 gears are way off.... So where am I going wrong?

Thanks for the help.

Tom

you are not wrong, but that is not a good idea. calculations do not take into account the extra weight of the tire. a samll difference in weight is a huge difference when it comes to a spinning mass

Plus you also need to take into acount the height difference which creates more wind resistance.

SEARCH more please...

It comes down to what you want. I didn't like the performance of my TJ w/ stock 3.07 and 30" tires. I ran those gears w/ my 32's and then regeared it to 3.55, then 3.73, and now 4.10, which just barely satisfies me. Both the other replies are right. You are adding frontal area to the vehicle with the lift and tires. Also more rotating weight and rolling resistance with the tires. These combine to make the same effective gear ratio(which you would have w/ 32's and 3.55's) ,well, less effective. But don't let anyone tell you you can't run it. It costs you nothing to run it with what you have. You can then decide if its suitable. You will take a hit in mileage if you don't regear, if that concerns you. Mine actually gets better mileage now than w/ the 3.73's. Gets the engine in its sweet spot.

ratherbcamping said:

Okay, I've been reading all the threads on gearing and I just don't get it.... Maybe most people think automatic transmission, but my XJ is a standard and I'm not going to change that (although I may upgrade to an AX-15 at some point). Here's my stupid logic so someone can explain where I’m going wrong.

First of all, this will be a toy for running around town and to the beach with the dogs. No serious rock climbing or trail riding planned. If that changes, then I'll rethink the tire size and gear ratio thing.

My 89 4.0/BA-10 XJ came from the factory with 3.07 gears. Now, I'm going to assume that my XJ came with 28" tires and the engineers that picked the 3.07 factory gears knew what they were doing. I have a donor 89 XJ 4.0/AW4 with 3.55 gears. The plan was to put these axles on my XJ.

The way I figure the math, I can run 32's and still be very close to the original design gear ratio. According to the posts I've read here, 3.55 gears are way off.... So where am I going wrong?

Thanks for the help.

Tom

Click to expand...

I think your right about the 32's and 3.55 being close to factory gearing at least with the 5sp. 31's would be a better choice if it's a street/daily driver since the factory gearing is tall anyways. I just went to 3.55's and 30's with the ax15 and i'm about 7% below stock XJ gearing. From what I know (correct me if wrong) the aw4 has a (2.80 1st) (1.53 2nd) (1.00 3rd) and (.75 od.) The 1st gear with the aw4 would be pretty tall with 3.55's and 32's. If your gonna run the 5sp, BA10 or AX15 the first gears are lower (3.83 ax15) and (3.39 BA10) that;s why they work ok with 3.07's. With 3.55's and the aw4 with 32's the 1st gear would be barely lower than a stock 5sp. XJ's 2nd gear. At 5000rpm with 3.55's and 32's, the aw4 1st would be close to 50mph. Too tall. That will make the 4.0 work alot harder to get things moving. Bigger/heavier tires and lift make it even worse. Hope this helps

FUNKYTEE5

I run 3.55's, an AX-15, and 30 inch tires up here in the Mts. I didn't like 31 inch tires with that combination, but altitude decreases horsepower noticably. I think 32 inch tires would not be a happy combination, even at sea level.

Fred

Thanks for the replies. This all makes sense to me. I actually plan to run 31's with the 3.55s and we'll see how it goes?

Not too worried about the highway or sorry gas mileage. This one is just a toy. I have a company truck (03 Silverado), a personal truck (another 03 Silverado) and my wife's 03 Liberty for highway travel. When you throw in my son's 95 YJ (he still lives at home) and my donor XJ, well...... we could pass as a car dealership. My insurance agent seems to be happy.

Thanks again,
Tom

ratherbcamping said:

My 89 4.0/BA-10 XJ came from the factory with 3.07 gears. Now, I'm going to assume that my XJ came with 28" tires and the engineers that picked the 3.07 factory gears knew what they were doing. I have a donor 89 XJ 4.0/AW4 with 3.55 gears. The plan was to put these axles on my XJ.

The way I figure the math, I can run 32's and still be very close to the original design gear ratio. According to the posts I've read here, 3.55 gears are way off.... So where am I going wrong?

Click to expand...

Your logic is impeccable, but it leads you to an incorrect assumption.

Yes, the engineers did know what they were doing -- they were designing a vehicle to achieve the best possible result on a federal emissions and fuel mileage test that was run on a dynamometer, which excluded any effects of wind speed (aerodynamic drag). I have an '88 4.0L with the 5-speed. Highway cruise is around 1600 RPM in 5th gear on stock tires, which is well below the torque peak of the engine, and thus not really producing optimum operation. For real-world operation on stock tires, 3.55s or even 3.73s would be a better choice, getting the highway cruise up to somewhere in the 2200 to 2500 RPM range.

For what it's worth, I also have an '88 Comanche (MJ) 5-speed. I run that on 31x10.50-15 tires and I have it regeared with 3.73s. In terms of road speed to RPM, it is EXACTLY the same overall final drive ratio as an XJ on stock tires with 3.55 gears.

For most people running 32s, probably 4.56 gears would be about right. If you really don't plan to do any rock crawling, and you will spend a lot of time on the road with it, then perhaps 4.10s will be marginally acceptable. 3.55s will be a waste of time, effort, and money.