Originally Posted by sonoronos
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
Yes all speedos are off very slightly but with new age electronics better than ever a DRB test at your Dealer will put this to rest. Have dealer perform scan test. Changing tire sizes requires re-calibration.
I buy all the speedguns for City I work for and all new units are factory calibrated, no recalibration required and only go astray if they are dropped.
Some previous Generation speedguns a few years ago would indicate a tree was going 120KPH when pointed at it, and other test was it would lock on to larger unit, howver with new units use laser tagging ends this.
The onus lies on honesty of Police Officer clearing previous offender,as if not cleared can continue tagging speeders with same reading as long as not reset. Even with the badge, some are not very honest and quota fillers.