Originally Posted by Texasscout

Try this one. You load in what you have now on one side and what you want on the other. It will tell you how much your speedometer will be off. I used it to go from 18” wheels to 19” wheels and keep my speedo the same. You do need to know the “offset” of your wheels. The coupe is usually +15-17 I think. The sedan is +45

http://www.rimsntires.com/specspro.jsp