in Europe, manufacturers must not allow a speedometer to under-read and the maximum permissable overread is by a calculation (this was discussed on a professional pilot forum i use).
by law in the UK we are allowed 110% +4
yes, i know it's europe, but the cars are designed and built to be sold across continents
for those interested in the legalities it's here:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:1975:196:0001:0005:EN:PDF
honda got drawn up on the over-read in US, here:
Odo Uh-Oh: Honda extending warranties on 6 million cars