Since you don't have GPS, go to their online calculator. Select "ESTIMATED." Enter stock front and rear sprocket teeth, to your current sprocket teeth. Then below that enter your wheel size. Then get the -.**% number and program that into your speedohealer. They factor in a decent amount of factory error too, so it'll be pretty close. Then whenever you get a GPS, reset your speedohealer and do it over with GPS and then use their "PRECISE" selection for their calculator. That's the best instruction we can give man. Or....go to an area and hold your bike as good as you can at 35mph or anywhere above really well. Have a buddy hold a radar gun on you 100% perfect and then get the number from him what his gun read.