Got a ticket once for going 75 in a 65 zone. I argued with the officer. He told me cars sometimes have bigger wheels/ tires put on them by a previous owners. Sure enough I had 16" tires on my car which was supposed to have 15". I t made for a better ride but it affects the speedometer. Looking back I was always passing people but I figured I was always pushing the speed limit. $134.00 later I learned my lesson. This happened in the early 90's so Im not sure if it still applies, just thought I'd throw it out there.
Yeah, pretty sure the speedometers are calibrated for the original sized tires.