The law on spedoomnetrs accuracy is set out in United Nations "UNIFORM PROVISIONS CONCERNING THE APPROVAL OF VEHICLES WITH
REGARD TO THE SPEEDOMETER EQUIPMENT INCLUDING ITS INSTALLATION" See:
https://www.unece.org/fileadmin/DAM/...gs/r039r1e.pdf
The speedo may have up to 10%+/- accuracy, but may NEVER underread. So they always overread.
John