I'm thinking there must be some others who are looking at how accurate GPS units are for measuring speed. So here's a bit of yahoogling on the idea. I contend that for a vehicle the most accurate measurement is going to be a properly calibrated measurement directly off the wheel. But after reading some of these I see there's some variables even when measuring right off the wheel. Knowing how many revolutions the wheel makes only tells you the speed if you accurately know the wheel's diameter, and then only if you can adjust the speedometer so it knows the proper wheel diameter.
http://continuouswave.com/ubb/Forum6/HTML/001066.html -- two boats drag racing, one GPS showing 38.5 to 39-MPH the other about 44-MPH
http://www.ncbi.nlm.nih.gov/pubmed/15519597 -- National Institute of Health study which concluded that bicycle speed measurements are pretty darn accurate with most GPS so long as you're in a straight line travel. Curvy travel is less accurate.
http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=29315 -- An observation that GPS tends to read 3mph different from a car speedometer -- and some guesswork as to why -- Such as wheel size variation due to wear and tear
http://forums.groundspeak.com/GC/index.php?showtopic=111969 -- a 5 miles/hr discrepency, and other guesswork as to why. One statement is legal requirements are such that speedometers can read high, but they cannot read low.
http://www.pocketgpsworld.com/modules.php?file=viewtopic&name=Forums&t=22561 -- General question, 'how accurate are they'? "The GPS manufacturers generally quote 0.1 MPH accuracy on flat ground, at constant speed and direction for several seconds and with a good sat lock. Yes, most car speedos will read up to 10% high (you wouldn't want it to read too low, would you?)"
http://www.4x4community.co.za/forum/showthread.php?t=10714 -- "In my bakkie (FORD) if I drive 130km/h on the Speedo the GPS shows +-116km/h, Bakkie showing 150km/h, GPS +-135km/h Bakkie 175km/h GPS 152km/h (top speed of beloved Frikkie die Ford)"