A dynometer is not a scan tool. It is a device that the car's drive wheels are placed on a large drum or sometimes two small drums. the drums spin with the wheels, and the speed of the turning drums shows the real ground speed of the tires. They are very accurate is calibrated correctly. You might look in the yellow pages for a radio shop that does speedometer work. They usually have the same type of dynometers to check vehicle speed. Then if the dealer's and the independent shops numbers match, case closed, if not you can get a certified copy from the speedo shop to use against the dealer.

Yes, the main members are in Europe, but Japan, Australia and New Zealand are also the members.
A regulation is discussed at World Forum for Harmonization of Vehicles Regulations(WP.29) and the US and Canada also attending the forum.
http://www.unece.org/trans/main/welcwp29.htm

I don't know how the US government is treating this, but there is no good reason to against the regulations.
I believe it is nice to set such standard regulations worldwide.

Ken@Japan

I know that the thread is basically over, but I thought I'd add two data points. I have about a 3mph difference between GPS and speedometer on my TCH. On my previous car (2003 Corolla CE) the divergence was about 2mph.