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If you do a search, the info is out there showing tire sizes do little to change the already inaccurate speedo. Find a streach of highway with mile markers. drive 60mph for 10 markers. Time yourself it should take 6 min. if your going 60 mph when you start. Callibrating speedos isn't a cheap while you wait operation. The water temp. gage is the one to worry about. Its function is cold--OK--too late!