If you have a GPS, and tachometer, you could just note the rpm's at 60mph then look up your tire size as to how many revs per mile it turns. For instance, at 60mph, if you have 11R-24.5 that turn 476rpm and the engine is turning 1585rpm, then divide 1585 by 476 and you come up with 3.33. Or you can lift up one tire and count the drive shaft revolutions. If only one wheel is up, it will be double. Good Luck, TomC