You need to know the gear ratio and the tire diameter. Here is a post on how to find the gear ratio. https://board.moparts.org/ubbthreads/ubbt...gear-ratio.html

Here is a chart you can use to figure out the correct gear. Just see what an inch of tire diameter makes in gear teeth with your gear ratio and go from there.
http://www.moparts.org/Tech/tech/pages/speedo.html