I feel like an idiot for asking this question considering the work I am doing on my Jeep, but I need to know what speedo gear to swap in for 42s w/ 5.38s. If it matters it is a 231/300 doubler w/ the 300 getting the speedo.

I tried looking for a decent calculator, but everyone that I found is just a stupid graph with tire sizes up to 35s.

newGear/newRingAndPinion = oldGear/oldRingAndPinion

Solve ratio for newGear to determine the proper tooth count for the speedometer gear

Check the write up.

http://www.ilovejeeps.com/forums/showthread.php?threadid=11657

Or this one;

http://www.4x4xplor.com/speedo.html