LCP

It was a 1.5mph difference at 100mph, and about 3-4mph by 150mph. What I failed to account for, that I now think Audi engineers did account for, is that the diameter of a tire increases as the rotational speed, and centripital forces, increase. A car acceleration software program I have estimated this in its default settings as 0.1% per 10mph, but in all likelihood, I think the equation is not linear, but exponential (since rotational forces are involved), so that the Audi speedometer may be accurate after all. Most people with GPS systems have stated it is accurate.

smallTTs

time between mile markers on interstate. Use the A-pillar as a timing reference point. Divide 3600 by the time in seconds (for one mile) to get average speed over that mile.

With 2/10th second timing accuracy, your are within 1/2% at 80 mph.

If you're gutsy, you could try higher speeds. Please use a passenger to do your timing at high speeds.

Dr.Aronis

GET REAL...just keep driving at 135 until a cop pulls you over, then ask him what your speed was...

LOL..

smallTTs

LOL. There are a lot of places where you'd run out of gas first. West Texas, for example.