Spedometer/Computer Speed Discrepancy

["Khan M. Klatt" <kmk@pacificrim.net>]

OK, check this out.

My friend drives a Corrado VR6, and he and I (driving my Jetta GLX) both
get on the freeway and peg our cars at 60mph w/ cruise control.

We both checked our computers, and reset the computer's "Average mph"
guage, and both cars showed 57mph.

I have stock wheels and tires on my car, and he bought VW wheels to replace
his BBS wheels, but is using the same stock tires (or the same type of
tires as stock).

It seems this speed discrepancy exists at all speeds-- approximately 3mph
difference between speedometer and computer.

So, is there anyone out there that has facts or figures that would indicate
which one is accurate and which one is not, and for what reason?

I would tend to think the computer is accurate, but I want some hard facts.
Any mechanics or engineers out there that know why this discrepancy exists?
How about having it fixed?

If the computer is wrong, does that affect the miles per gallon and time of
trip calculations, etc.?

Has anyone tested the accuracy of the temperature guage?

- -Khan