Dashcam for motorcycle

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Answer; the most accurate in the right circumstances; straight road, no big skyscrapers.

Gps calculates the distance between imaginary "dots"on the road, at very precise intervals, the more dots in a straight line the more accurate the result will be.
Similar to counting every ten light poles with a stopwatch.
How accurate? it presents a cruise missile into a bathroom window 1000 km away consistently provided the window does not move.

The whole world's telecom structure ( how you get the internet ) relies on the accuracy of the atomic clock in n.i.s.t. in Fort Collins Colorado;
vibrations, or frequency) is 9.4×10−19, the best of any clock worldwide. This means the logic clock would now neither gain nor lose one second in 33 billion years, which is about two-and-a-half times the estimated age of the universe.

The way to prove this is to drive with preferably more than two gps systems simultaneously and observe the difference ( if you can measure it ) in speed in various locations, fortunately someone has already done this, fast forward to 9 minutes into the video, our friend is not up to date with everything but he does the test correctly.

https://www.youtube.com/watch?v=xo_bcB0u2IM

The secret is the fact that a gps calibrates itself and a laser or radar instrument needs periodic checks to verify it's accuracy, some officers have the mistaken belief that a gps needs to be calibrated too, if they say that ask them how that's done....