56. Renties Ypres Rally Belgium 2021

[Mirek]

And even the speed guns of the police have a rather big margin of error.

Only gps speeds should really be trusted.

Police radars are usually under +/- 3 km/h (at least here). That's not a big margin of error and it's not worse than GPS.

GPS speed is not accurate. GPS has large latency, limited position accuracy and very bad accuracy in vertical (i.e. when the road leads uphill or downhill it can not correctly count the real distance you drove during the signal period). It's accurate basically only when the speed is high and constant over a certain period of time. For dynamic driving where you accelerate and brake and nothing in between it is useless. It is also useless when you drive really slow. Basically the faster you move the lower GPS error (because it comes from the position accuracy which is usually around +/- 6 meters). When you walk GPS gives total bullshit speed numbers which is normal. With rising speed that bullshit gets more reasonable.

The most accurate measurement of speed is via optical or radar sensors mounted on the car itself (those tubes pointed on the ground you see during testing). The stationary radars are second best, if well calibrated (there are more precise ones than those used by police as well).