Awwww why didnt I think of that!?

:-DCongrats!

Here speedometers are nearly always set to over read. Usually by about 10mph at that sort of speed. Use a GPS system to prove this.

I don't know how accurate the speedo in the TC may be, but overreading is a great feature for manufacturers . . . it makes the car look quicker, more fuel efficient, and more reliable (racks up miles more quickly.)

He could just get a good estimate with math . . . he knows his engine rpm and could look up the gearing and then make a couple of estimates for different tire diameters (as it varies a bit at speed) and I'm sure he'd atleast get a good idea of how accurate his reading was.