We have found this calculator to be most accurate:
This is the calculator we use to figure your revolutions per mile: Tire size calculator
It works with the metric system. If you have your tire size in the standard system, please figure the metric values first with this calculator and then go on to determine your revolutions per mile with the above calculator.
Metric Tire Conversion Calculator - Red Rock 4x4
We tested the revs from the manufacturers site, as opposed to our calculations with Powerdog and have found the Powerdog feels like better shifts. This is something you can try also.
Always remember if you are going to set your revs per mile, you must also set your gear ratio.
There are two differences when you set it yourself, in the options menu, before you load a tune onto the truck, as opposed to your tuner setting it for you.
1.) You can only set it in increments of 5, i.e., 575, 580, 585 ... Your tuner can set any number.
2.) Every time you load that file you need to reset your gear ratio and revolutions per mile. If it is set in your tune, by your tuner, it is always there and you don't need to reset it.
Hope this helps.