Are you running transaxle or live axle? If your running a transaxle do this--
Take you rear pulley and divide it into your front pulley, multiply that times your engine RPMs, divide that by top gear tranny ratio(12 if its 12:1) save that number.Then tire size ( 14", 15", 16") X 3.14, take that number X the saved number = inches per min. Multiply that times 60 = inches per hour, divide that by 12 = feet per hour, divide that by 5280 = Miles Per Hour.