- 2.5L - 4 cylinder engine
- 5-speed AX-5 transmission
- 32x10.5" BFG AT tires
- 4.56 ratio differentials
When looking at trying to balance differential gear ratios and tire size, you have to decide which parameter is most important to you.
1. Low-end torque (ability to climb trees and rocks)
2. Acceleration (ability to leap into interstate traffic)
3. High-end power (ability to climb hills at highway speeds)
4. Gas mileage (ability to go farther and operate cheaply)
Playing this game is like squeezing a balloon... you tighten down in one place but it blows up somewhere else.
My journey started out when I decided my jeep would look better with bigger tires. The jeep started with the original size tires (about 27" dia). The existing tires were over 10 years old and had cracks in the sidewalls, so it was time for new tires anyway. So I invested in a 5 new rims and 5 brand new BFG 32x10.5" tires (the spare was original).
The Jeep still rode rough, and the tires would rub the fender wells every now and then because it needed shocks and the springs had sagged. So I invested in a 4" lift. That was a major improvement in comfort of ride.
However, I drove the Jeep to visit family in Mississippi and could really tell the difference in how it performed on the highway. I got 12-13 mpg trying to drive 60 - 65 mph on the interstate. Almost any uphill grade or headwind would force me to drop out of 5th gear just to try maintaining speed. I was contemplating going back to small tires.
As you have seen people do on here, the answer I got was re-gear. So I did my research and decided to re-gear to 4.56 ratio. I am still weak on the high-end power, but not as bad as I was before re-gearing. Mileage actually improved to around 18 mpg driving around 65 mph using low-ethanol fuel.
Last year, when I went on vacation in Colorado, I was only able to go 20-30 mph going uphill on the Million Dollar Highway. This year, I was in the 35-40 mph range. I suspect 4.88 ratio gears would improve this, but not sure how it will impact everything else.
As for RPMs, I pull somewhere around 2900 RPM in 5th gear at 70 mph. I estimate that I would be pulling around 3200 RPM with 4.88 gears. That is not too bad in itself... but I suspect I would be maxing out RPMs getting up to speed in the lower gears.