Originally Posted by dyezak
So I used my brother's GPS and found that my speedometer was indeed off based on it, but thought that the GPS couldn't be that accurate of a measurement. Then tonight on my way home from work I passed one of those things on the side of the road to remind you of your speed. I looked and it said I was doing 38 and my speedo showed 34. So it seems as though my speedo is 10% slow (38/34 and 78/71)
GPS is accurate to whatever the satellite fix radius is present. On average, this is 30 meters, but can be as fine as 3 meters (without differential GPS.)
In any case, a properly calibrated police radar speed gun is significantly more accurate than standard, good old-fashioned non-DGPS. It is possible that the speedometer is inaccurate, but without a radar gun, the most accurate way to determine the car's real speed is to time how long it takes to travel down a straight stretch of road a given distance at a given speed marked by your speedometer.
A simple stopwatch and some good old fashioned math could be more accurate than the radar gun
