You need this info first:

      The height of your tire, not size.

      The gear ratio of each overdrive gear of your transmission

      The speed you want to drive at, usually 75mph 

 

How to use the chart.

      Pick the tire hiegth, pick your gear ratio. Right down that RPM number.

      Divide that number by 60. Write down that number.

      Multiply that number by any speed, 50, 60, 75 etc.

      This what your engine is turning with a no overdrive transmission.

 

Now for those Overdrive Transmissions:

      Go to Google and type "gear ratios for ___________transmission. I used a T56 Ford 6 speed.

      Take your RPM at your selected speed and write down.

      Multiply that RPM number ( 1st chart ) with the ratio ( 2nd Chart ) and you get your RPM at that given speed.