i have heard different stories for each, and just wanted to know which gives a better top speed read out? i would think gps due to your position aligned with the satelight. keeping in mind multiple runs have to be made with both, as each may be able to give a read out that may be off in a single occasion. what is everyone using, and thier opinions of why thiers is better?

GPS is a good tool but is not the most accurate. I have noticed if I travel from 0 to 70 in my truck the GPS reads light for 3 to 5 seconds before it matchs the speed of the truck. Then it’s a little off. My concern is the max testing area I have is about ¾ of mile long. Most of the time its ½ or less. So my actual top speed is not 100 % right. It is consistent for the most part and a great inexpensive tool. I hope to compare it to radar this weekend and will post results to this thread. The GPS is use is the Garmen E-Trex, the yellow one.

Radar hands down. GPS is subject to too many variables. Don't forget GPS was not designed as a speedometer but as a location device, radar was designed specifically for speed readings.

For accuracy, they are within a mph of each other. the beauty of a GPS is you can tune and test With no help from a second party.. It costs less too. You simply need to purchase a unit with faster updates. I use the yellow Garmin etrex. I do a pass and then check for max. speed. I also am checking rpm with my pet tachometer at the same time.