C&D: Mazdaspeed 3 vs. Volkswagen GTI

[Tave]

Is the speedometer error caused by tire expansion at higher speeds?

I have no idea. Like I said, that's just what I've been told.

I mean, unless the road was significantly sloped, it wouldn't make much of a difference, but wouldn't GPS just measure location change assuming the Earth is flat?

For example, say you're driving up edge C. Wouldn't the GPS just be able to see change of position on edge B?

Ah, now here's a question I can answer.

3D. GPS does everything in 3D. Sometimes the altitude is a little off though, because Earth is not a perfect ellipsoid.

Yes. Moreover, biggest error in GPS positioning is in the vertical. GPS does not do elevations nearly as well as lat-long. Even our $50,000 machines at work need a lot of time on a point to get an accurate reading. It's getting better, but it's still pretty rough. In fact we still use a total station or digital gun for level loops.

[The Pirate]

Where will you see a dropoff in speedometer accuracy?  Comparing against radar signs, my Passat was optimistic up to 3mph at 80, and my Jetta is spot on at 80. 

Per my manufacturing processes professor, a dial gauge is most accurate in the middle portions of it's travel.  At either extreme (min and max), it's going to be less accurate. 

Also, what r0tor says.