I recently busted the pinion in my 8.25. I have a 4.0L 5 speed, all stock susp. and 235/75r15 tires. I had to pull another rear axle from an automatic xj, giving me lower 3.55 gears. Since my gears are lower now, I'm only hitting about 60-65mph in 5th gear, at 2200ish rpm's. Would a 31" tire upgrade help balance out my rpm's and let me go faster, or should would I have to go a bit bigger? Not looking to break the bank, 3" lift is about as high as I can go right now.

 

Gear sets are designed for a good balance between low end acceleration and top end power. A lower gear set will increase your acceleration but you will lose top end power (highway cruising/passing) a higher gear set will make you accelerate slower but have better top end power. The 3.55 in a manual equipped XJ is like us autos switching to 4.10s. So in short, either grab some 31s or find the original 3.07s for your axle. Good luck.

 

The 3.55s should work just fine for your 235s, I came up with 2119 rpm at 65 mph which is great. I come up with 2445 rpms at 75 mph and the 4.0 can easily run those rpms all day long.
If you live somewhere where almost all your driving is on flat roads maybe you should look for the stock 3.07s, but for the most part the 3.55s will make your Cherokee perform better, because in the mountains where I live the stock gears sucked.

 

Depends on what you mean by "let you go faster."

Did you switch out the speedo gear or are you going off GPS for your speed reading? If nothing was changed, your speed should be reading the same RPM to MPH even though you are physically going slower. Anyway, your gearing has gone down 15%. A 33" tire would return you to your previous stats, but the increased resistance from weight and wind resistance from the lift will probably make it feel a little more sluggish.

A 31" will kind of split it in half. The 4.0 is pretty comfortable cruising all through the 2000-2800 range, so you might as well find a combination you like the feel of an run with it. Gas mileage seems to stay about the same throughout the RPM range as you trade lugging for higher RPM.

Be sure to get a matching ratio in the front!

 

I have a 93 2dr 5spd on 4" of lift. I have 3.55 gears and 31s. Going 65 I run at 1950 and have plenty of power high and low

 

I have a 99 with 6" lift and 35's, 5spd, on 3.50 (Ford gear ratio)...It does fine. However 5th gear is not useful on hills at all.

2000 RPM @ 70 MPH
Speedo is just about spot on with no change to the speedo gear. Although the previous owner had 31s and may have changed it for those.

 

Ax-15 as well?

 

If that was directed at me...yes, sir!