[High On Octane]

I disagree.  I find GPS speedometers VERY accurate.  No, you are not driving in a straight line, BUT the GPS has already been loaded with all the roads AND their mileage/distance.  So even if you're going through some twisties your GPS mini computer knows exactly how far you have traveled in what time.  Most GPS units calculate you speed every 1-3 seconds, so it is always calculating as you speed up or down no matter where you are.

As for your speedometer only being 3 mph off... Must be nice!  With the new shorter tires on my Trailblazer my speedo went from being 15mph fast to being 20mph fast.

Scottie

[D the D]

Agree with Scottie.  GPS is based on satelites, not ground stations.  The only thing blocking you is heavy rain/snow (very thick clouds) and tall buildings in a downtown area.  We put them in military aircraft as a backup system a decade ago where their accuracy is pretty close to the standard ring laser gyros and accelerometers, airliners use them, ships use them as primary navigation on open seas, our troops navigate with handheld and vehicle mounted units.  Oh, and the military aircraft  don't get more accurate GPS, they get two frequencies per satellite to help correct for ionosphere and atmospheric interference in geo-location.  It reduces the possibility of error, but is not more accurate per se, which is why it isn't the primary navigation device.  The speed calc of common GPS units is very good. {20 years of managing Training for Naval Aviators and Maintainers  }

[wildbill]

on both my c5's i found they were 10 kph or 6 mph fast at a genuine 100kph or 62 mph.
in fact once over 30 mph they both got a bit excited with the speed reading

[barenekd]

I once showed 92 on my G5. Didn't really believe it! With an 18T sprocket and going slightly down hill, perhaps it wasn't that far off. It usually only appeared to be +2 or 3.
Today, in fact, I showed 107 and climbing on the Goose. By my calculations with RPM/speed, the bike should only do 103. I was running out of room so I didn't get a chance to look at the tach.
Bare

[D the D]

My GPS said my Iron Barrel did a top speed of 72 once, but I have no idea what the speedometer indicated.  I didn't know until I got home and checked the trip stats for mileage.  I usually try to stay under 60 and that's not usually hard to do.  I had to have been going down hill with a stiff tail wind. 

[REdmonton]

Here are a couple of studies that discuss GPS speedometer accuracy.
http://nujournal.net/HighAccuracySpeed.pdf
http://www.gps-results.com/GPS_Speed.pdf
If the GPS is using doppler shift measurement then it is more accurate.
ScottieJ how do you know your GPS read out is accurate? What have you compared it to?
D the D I could understand that ships, airplanes and especially military aircraft has highly accurate GPS systems. Most of us don't have access that kind of technology. Often consumer based GPS units have accurate within 5 meters this could lead to some in built errors.

[D the D]

I won't argue it.  Buy a cheap GPS and you get a cheap GPS.