if you do a search for speed, there is plenty of advice on how to measure your speed. In short, GPS speed readings are at best, suspect. I once registered a speed of 73 knots on the GPS during the tybee.
A friend of mine also logged a rediculous speed whilst participating in the Fast Cat I.O.W race. Whilst having a few beers after the race we discovered his GPS was placed on a string around his neck, whilst tacking our gybing we reckoned it might have swung to a radical speed but are they really that accurate? I doubt it.
