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Speed readings obtained from a GPS unit is never going to be as accurate as the reading derived directly from the car, because GPS locations are only accurate to about 10ft (IINM).

Your car speed can be used in tandem with the relatively inaccurate location given by a GPS unit to give a better location reading. More info here: http://en.wikibooks.org/wiki/Robotics/Navigation/Localizatio...

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GPS extracts velocity data from the doppler shift of the signals, not by subtracting successive locations, so the 10ft accuracy doesn't come into play. My experience with GPS velocity data has been that it's more accurate than the average car's speedometer as long as you have a decent fix.

Of course, having poor signal in urban canyons or tunnels is the sort of thing I didn't think of, and that's where getting data from the car would really be handy.