Hope this helps.

If you're interested the math is as follows, this will initially be calculating for 60MPH, but will be further extrapolated.

Determine the circumference of the tire, Diameter x Pi (3.14), not going be exact but will get you in the ballpark.

That's how many inches it has to roll for one revolution.

There are 63,360" in a mile (5280' x 12).

Divide that by your tire circumference to get wheel revolutions per mile.

Since 60MPH is one mile per minute, it is conveniently also going be the wheel RPM.

Multiply that number by your gear ratio and that's your engine RPM (assuming 1:1 4th gear, no overdrive).

Vary the gear ratio to get into the RPM range your looking for.

Divide the engine RPM x 60 and you'll get a factor you can multiply by what ever MPH you want to determine engine RPM at that speed.

I have 2.5 on 32" tires w/4.88s, my multiplication factor is 51, a stock YJ on 27s w/4.10s is the same, perfect match (on paper).

So even the speedometer is correct, or pretty close to it.