Ah, but I have never had a bad point on my GPS. I upload my tracks to my PC and plot them in both National Geographic's TOPO! series and Garmin's Maps & Waypoints. I can see my speed increasing as I get further down the mountain, peaking, then slowing down as I bottom out. And I've done that route multiple times so I'm very confident I have an accurate number.

I use Garmin's GPSMap 60CS and have never had a problem with it, even in rain.

And, FWIW, I also reduced the circumference measurement on my cyclocomputer. I sat on my bike and rolled forward as my old man marked off a full rotation, but, according to my GPS, it was still a bit too large. So I dropped it about 10+mm and came out with the exact same top speed as the GPS.