I just want to say this, because apparently everyone thinks the opposite.
Never consider your GPS speed to be absolutely accurate. It isn't. The US government introduces a bit of error into the GPS system for civilians. This is so a civilian can't use GPS to direct a weapon at a specific target. The error is around 10 meters (notice next time you exit how your GPS might not recognize immediately that you left the highway), which is more than accurate enough for most things, but since the error is randomized, you can't count on your GPS calculating your speed accurately.
If your speedo says 70 MPH and your GPS says 75 MPH, you're probably going 71-72 MPH. If your speedo says 70 MPH and your GPS says 72 MPH, you're probably going 70 MPH.
If you can't use GPS to set your speedo correctly, what can you use? Don't use radar, because it's also prone to error (and it gets exponentially worse the further you get away from the radar beam, since the beam spreads.. a radar beam is only good for around 4 lanes of traffic, and after that it's the size of a car or two and not accurate). I think that a dyno might be able to measure the speed your wheels are actually traveling, and THAT is what I'd use to tune my speedo. If that's not an option, leave it alone.