The reply saying to rotate the rear tires and count the driveshaft revolutions works, or if you are not sure if you want to change and go to another ratio check out this website below. Pick the trans. you have, tire size, gears, and click.......you have the RPM at any given speed. There is a lot of good info on the site..... Hope it helps.
http://www.paladinmicro.com/pmicro.p...nGearCalc.htm#