Quote Originally Posted by the evil twin View Post
Actually that is wrong, it is not +/- 10%.

The subject has been done to death on this forum but to save the searching... ADR's since about 25 years ago require that speedos cannot underead at all. IE 0 tolerance.

Speedos are allowed to overread by up to 10% plus 4 KPH. Most manuf will have the speedo set so it overreads even with the largest size tyre as per the placard.

Therefore if you are driving to your speedo you cannot be speeding. If you change your tyre size or ratios then the onus is on you as the person who modified the vehicle to adjust the speedo or wear the consequences.
If thats the case then why in every training ive been to say 10% either way, manafacturers themself even state that varient in manuals.