Speedometer accuracy

GPS is good for getting your average speed on a trip but I wouldn't use it to check the accuracy of your speedometer. GPS won't tell you how fast your tires are rotating at this moment. ajengli was right about tire wear affecting the speedometer. Try taking 28 inch tires off the Jeep and putting 35 inch tires on. GPS gives you your current position, it calculates speed by how quickly you change that position. GPS positioning is only accurate to within 10 meters or so; so you could move 10 meters in an instant and the GPS could read zero speed.

How does GPS work? There are 26 satellites in orbit, each one has a cesium clock and broadcasts the time via radio waves. Your GPS reciever checks the time broadcast compared to the time the signal was recieved and calculates the distance from the satellite. (the radio waves move at the speed of light which is a constant and the GPS has a built in Almanac so it knows where the satellite is supposed to be). You need line-of-sight to at least 3 satellites to get a fix. (3 spheres intersect at 2 points, one is your position and the other is out in space so the GPS reciever rejects that position) So a lot of the inaccuracies with GPS stem from the orbit of the satellite varying from the GPS receiver's almanac and the accuracy of the GPS receivers clock. So if you ever wondered what the difference between a $100 GPS and a $1,000 GPS was, the $1,000 GPS has a more accurate clock.