I know most cars over read but I've never seen 10%. Porsche are usually spot on.
I'm sure if its over reading then the milometer must be doing the same???
Boxsters (986) are renowned for over-reading speedometers. My S was virtually 10% over from a few mph up to, well, certainly 70.
As I said, can't comment on the electronic speedos, but certainly the mechanical are two independent instuments. The mileage recorder is a fixed gear drive (usually of the worm type) from the end of the speedo cable, where the speed reading is taken from a magnetic pickup (how they worked was covered in an edition of The Morgan World a few years ago). The only way to tell whether your mileometer is correct is to cover a fixed distance and compare distances. My +4, which had an electronic speedo, used to give different mileages between trip meter and odometer - so the measurement can be altered.
As far as the rules go, I understood over read only was allowed - up to 10%, but a google search on the subject is a nightmare. The info should, I guess, be in the Construction & Use regs, somewhere... TG did a test on a few cars some years ago - Mondeo read 29 at a true 30 and was declared illegal, but that's TG... Part of the 10% over read is to allow for tyre wear, between new and full worn, though with today's thinner tread I doubt it is necessary to allow so much.
Cheers,
