A few weeks ago, I had our GPS in the Beetle for a couple of days. The GPS has a spedometer feature on it, and I observed that the speed the GPS reported was lower than the speed on the car's spedometer. In fact, it was about 5% off.
I didn't know which one to trust, but then I observed that the radar speed reader that the police put out also put me at about 5% slower than what my spedometer read. So I'm thinking my spedometer reads 5% faster than the actual speed.
This explains the times when I've been driving along at the speed limit and other cars have come along and driven right behind me, like I'm driving too slow. I guess I really was going about 5% too slow even when I thought I was going the speed limit.