well using stock at a 195-65-15 and going to a 215-35-18 like you have in your profile its gonna be off by 4.7 % when your car says 60 its actuly going 57.5
go with a 225-40 next time lol
are you just trying to get a dyno to see power ? if so wheel size shouldn't have a effect on it other then more rotating mass sure our car is reading wrong but the dyno doesn't read off your car..
just go some where and don't say anything lol just get the dyno if the numbers seem that far off (in which im wrong about the tire thing but i don't think i am) then jsut take 4.7% off
i ahve to do this with my gas milage cause i have slightly larger tires used to get 300 miles to tank now i get 260 but when i take in the percent diffrenceim back to my 300