This is an excel spreadsheet I put together a little while ago whilst looking at the effect changing tyres would have on the general driveability of the JK.

Over time the sheet expanded to include petrol, diesel, auto and manual with all variations of gearboxes. Simply enter the tyre size and select the diff ratio and transfer case ratio then type in the speed and the approx RPM will be shown.

After mucking around a bit and then testing against real life, I discovered the rolling diameter of a tyre is around 96% of the measured diameter. So I have changed the calculations to use this instead (basically the rolling diameter of a tyre is approx the diameter of the steel belt which sits just under the rubber at the base of the tread, ie a bald tyre).

I have now successfully converted the Excel spreadsheet to an online version that you see below (I have noticed it can take a little delay to show on older/slower computers so please be patient if the below section is blank).

Hope it helps you!