Don't use this chart it is for a manual transmission with a 1 to 1 final ratio.

If you use the grimmjeeper.com calculator you will be dead on as to tire size and rpm traveling at whatever speed you select. For a lifted rig running larger than stock tires you'll need to turn a few more rpms to overcome the extra drag and the heavier tires.

http://www.grimmjeeper.com/gears.html

 

Old Tire Diameter / New Tire Diameter * Current Gear Ratio = Effective Gear Ratio. (Tire diameter must be in inches) The trick is to input different gear ratios (Current Gear Ratio) into the formula untill your Effective Gear Ratio is as close to stock as possible. Ex: if your Xj was equipped with 3.55 gearing, then you want the final math to be as close to that as possible. Therefore your Xj will think and act just like it was turning a set of stock wheels/tires.

 

I just regeared from 3.55's to 4.56's with lockers. Lifted 4 1/2' running 33x12:50x15's lots of drag worked out to be the best gear for me.

 

Ok Fred, your our guinea pig for this example. Lol Now, 27 ÷ 33 × 4.56 = 3.73. Your Xj acts just like a stock automatic with 3.73 gearing (available on the towing package models). Thats a good gear for your setup now. Now lets say you kept the stock 3.55 gearing, 27 ÷ 33 × 3.55 = 2.90 this is way higher than the stock 3.55 gearing to where your motor will have to work way harder to turn those 33s. Do you guys now understand the math behind proper regearing? As I stated earlier, the goal is to choose the right gearing to get your Xj to think it still has the stock tire size and keep the powerband in a usable RPM range.