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"My 81 has CC 3.31 Trac-Lok, and I have 2.5” lift with Rocky Road YJ spring conversion. I’m running 33x12.5 15R, and I have the I6 258 4.2. When going uphill she boggs down, and loses speed. I end up letting folks pass on mountain roads. What gears will allow me to get better MPG and have 4th pull me up the hill?"

Is this the question? 3.31 is pretty tall gearing for 33" rubber and the 258. Lowering it to at least 4.11 or lower would make a huge difference. I assume you've got the 4 speed still. Too low and you'll feel like you're going to throw a rod on the highway. 4.11 with the 1:1 4 speed will generally give you about half the rpm of your speed (60 mph= 3000 rpm, 50 mpg= 2500 rpm).i
Changing gearing is pretty involved. Smaller rubber and more usable horsepower (say with a sniper fuel injection, HEI and 2.5" exhaust) could give you the results you're after with much less work.
 
I have the exact same set up. I’m getting ready to swap in 4.56 tomorrow. If I remember I’ll let you know. I will probably move to a 5 spend next year.
 
My ‘82 has 4.10 gears and has a 232 I-6 from an AMC Spirit (single barrel carb). I was running 31” tires. Even with 4.10 gears it could not hold speed in the hill around Austin. I would need to down shift and still lose speed to the tune of 70 drop to 40. But that engine was gutless and worn out.

Your issue might be two fold. 3.31 gears are really tall and it will take a lot of power to hold speed. So, gears and power are the two issues you will probably want to address.

If you want to know what rpm you will be running at any given speed with a specific setup, go over to grim jeeper’s page.


Input the data and compare gear ratios to find your ideal setup.
 
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