I wouldn't be so certain that the error in speed measurements is due to the speedometer. Car speedometers work by measuring wheel rotations, using knowledge of

I wouldn't be so certain that the error in speed measurements is due to the speedometer. Car speedometers work by measuring wheel rotations, using knowledge of the wheel diameter and calculating the speed. You can get errors when the wheel slips (very unlikely except at very low speeds), and when the wheel diameter is less or more than the assumption (level of tread wear, tyre inflation). For a given journey (and over a period of days) this error will be pretty constant and linear in nature - the faster you go, the greater the error by some factor. Regulations mean that it's much more likely to over-report than under-report the speed, but I'd assume that this would be by a fairly constant amount. GPS speed is calculated by distance travelled between 2 positions in a given time. The problem is that GPS positions aren't completely accurate. Rounding errors, atmospheric effects and signal reflection can all affect the accuracy of the position. At low speeds, these errors can be as big as the distance travelled between measurements. The GPS speed may not take into account altitude (which is considerably less accurate on consumer GPS receivers), and so would under-report speed going up or down hills. It will also assume perfectly straight lines between points, while your car may in fact be travelling a less straight path. There's also likely to be a degree of averaging involved in the calculation, which would filter out small speed changes reflected in your speedometer data. Your plot shows times when the GPS speed was as high as 20 mph when the car speedometer was reporting 0 mph. No car with a functioning speedometer will report 0 mph when it's actually moving at over 20 mph. The convergence at higher speeds is probably more to do with the accuracy of GPS speed measurements at high speed than it is to do with the accuracy of a car speedometer at high speeds.