Considering making modifications to your Jeep? Whether it is changing an axle or going with different tires this chart is helpful in finding the right Tire Size to Gear Ratio that you need to achieve the optimal performance for your particular application.
Also, when making altering your factory gear ratio or wheel size your speedometer and odometer may read incorrect. Make the necessary changes to your speedo by replacing your Jeep Speedometer Gear.
A basic method to calculate is to mutiply .12 by your tire diameter. (.12 X 38" = 4.56)
The below table can be used to get a rough idea on gear ratios. The colors represent ideal RPM's at highway speeds (65). These calculations are assuming a manual transmission with a 1:1 ratio. If you drive an automatic your RPMs will be slightly higher, and the opposite is true if you have overdrive (your RPMs will be slightly lower).