My 1992 Jeep Cherokee is a 4.0 AX15 NP231 with 3.55 gears and 235/75R15 tires. Thats the same tire size I use for all my Eagles. It requires slight changes inside the fender flares, but its the largest size you can put on without any external changes. I had two identical automatic station wagons with different axle ratios and there was a 4 mpg difference, but I do not think you will see that significant of a difference with a manual vehicle.
I found the gearing to be perfect. I got about 25 MPG on the highway at 75 mph, which is quite respectable for a Cherokee at that speed. At 75 mph I would have to often downshift into 4th to maintain my speed up hills. If I was to change it, I would actually go a next size lower 3.73s. If you go with 3.07s, your 5th gear will become useless and your first gear will be less desireable in traffic. I often start in second in that Jeep but appreciate the low first there when I need it. Your engine needs to be turning at higher RPMs to put out enough power to maintain highway speeds. My opinion is to stick with the 3.55s.
I personally call them limited slips if they have a smooth clutch action, soft lockers if they are centrifugal units like the popular powertraxx models, and hard lockers if they are cable shifted or otherwise fully engaging lockers. A factory installed limited slip differential is the best option for most Eagles. A Ford Explorer axle is the best choice to get a limited slip and rear disc brakes. I'm using a Jeep Cherokee Dana 44 with factory limited slip and Jeep Grand Cherokee disc brakes, but thats hard and expensive to find in comparison to the Ford axles. The easiest axles to find are Chrysler 8.25" but I haven't seen many with a limited slip. They are much stronger than the Eagle axles because of the stiffer axle tubes. I still avoid them, since I won't buy a C clip axle after having three wheels fall off in the rear of an Eagle. The biggest advantage to the Dana 44 is that the wheel remains attached even if the axle shaft breaks.
I have a centrifugal unit in my Jeep Scrambler. Its effective, but it has a noticeable effect in curves. My sister refuses to drive it because of the soft locker. Their biggest advantage is that they can be installed quickly and easily. No change to the carrier is necessary. As a wheel loses traction and the wheel begins to spin much faster relative to the other, the centrifugal force locks the two halves together. It clunks together and often causes an audible tire screech. The clutches in a limited slip differential are much smoother and less likely to scare the driver, but in some ways that makes them less effective than a soft locker.
A hard locker is much more expensive and harder to install. The one I want to buy for my 401 5 speed SX4 is electric. Its a limited slip until you push the button and then the axles are fully locked together. My Rubicon has this option front and rear and I absolutely love them. Most of us would never install one since they're worth more than the average Eagle. I think a soft locker costs about 300 dollars and a hard locker 1200 to 2000 dollars. A limited slip differential is expensive unless you buy a whole axle with a factory unit already installed.
Topic: Traction Improvement Devices (Read 30023 times)