I have been using a small portable GPS unit while sailing my F18 boat. I have tested the GPS against odometeters and measured courses. I once competed in open class rallies and learned to verify odometers and even this fairly inexpesive model is accurate to about a 1.5% or less error, in measuring speed.

I had a friend attempt [color:"blue"] [/color] to measure the speed of my boat with a RADAR gun, but had difficulty getting accurate measurements from a fiberglass, kevlar, mylar, and aluminium target.

Another advantage in the portable GPS units, is choice of speed measurement displays. Most units allow KPH, Knots, or MPH display.

I usually have mine set on MPH, and have clocked a high speed of 31 MPH in 25 MPH winds.

Mike