I remember a bunch of upset people a few years ago at the shootout. They were complaining that their onboard gps units were reading much higher than the radar guns that the officials were using. I believe they have corrected a lot of it over the past few years. They moved the location and angle of the radar guns to get a more accurate reading.
Eddie

The radar gun only detects the component of the velocity in the direction of the gun. The error starts getting big fairly quickly as the gun gets off the track of the boat or you shoot from the boat at an object not straight ahead of the boat.

I calculated the error based on the distance from the boat course once after I got a miserable speed in a radar run but I can't find the spreadsheet. If I find it I will post.

Thanks for the post on this subject. I called livorsi, Todd, says; really accurate. I talked with a S.C. State Tropper, that instructs radar. He explained how the radar would usually show lower speeds but, not higher. If in question they shot you from the front radar and than the rear. There are incorrect readings you can get, but they train as you may well know for proper operation. They have GPS in their cars that reads exactly what the vehicles speed is on the radar gun. My wifes Gps in her SUV reads about 3 mph slower than the speedometer. Maybe the Radar from the State Tropper is for real and the GPS is reading 92, I guess I'll just wait until the props are lab finished and see if we get the 5400 rpm and 96 mph, then the Spoiled Brat, can take the boat and have it shot again by radar. First time I ask him about GPS recall, he was LOST. Thanks Mark