GPS and ground speed
Interesting- I purchased a hand held gps which is stated to be accurate to 0.1 knot,to check ground speed when sailing to take advantage of tides and current flows.Out of interest i thought I would check my nissan's speedo. To my surprise it was reading 5mph over on the car. the car is also fitted with sat -nav but only gives average speed. The average speed calculations for both gps was indentical.I have checked my colleages cars and all vary between 1-5 mph over read which I understand the manufactures adjust to protect liability.