rea

This.

GPS also clips the corners into segments of straight lines.  Like the circles displayed on a computer screen.  I see errors on my bicycle very often when tracking using my iPhone gps vs a calibrated speedometer that reads from the front wheel that I set up to the millimeter using a weighted roll out method.  

Max speed on the GPS units are usually full of error.

stop watch, interstate mile markers (2 or so), go 60 /thread

I set my speedometer calibrator to GPS so my speed is exactly accurate.  Mine adjusts it in like .0001mph increments though, so it's easy to get it exactly correct.

Both my GPS and the Radar displays by the side of the road always show a couple MPH slower than my speedometer.

Did you calibrate the speedometer with a rollout test or just assume that your tires match a book number?

All my cars have read high. I trust the GPS.

My wife's Subaru is dead on to the GPS. I think it is because it has the anti-collision thing and it needs accurate speed to calculate how long until impact.

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My Tacoma was 2 mph faster than the GPS. I got new tires and now it is about 4 mph faster. My bike is close to 5 mph


 

GPS for accuracy. Speedo for responsiveness to change in speed.

My GPS is usually 1 mph faster than my speedometer.  I use the GPS

GPS is probably more accurate for most situations but:



A GPS is speed over distance, it loses some accuracy do to changes in elevation. Boats are awesome for GPS speedos because the pitot tube on boats generally suck.



In a vehicle, imagine doing 50mph going up or down a large hill at a steep grade.



A speedometer has more room for error due to tire spin, tire size change etc. But if all match up it should be fairly accurate.



I would like SHOTAR to try an experiment to see how they compare side by side in the scenario above.



In general use one of my vehicles us spot on while the other is consistently 6mph faster than what the speedo shows so I assume the GPS is right; this is on my truck with different than original tires on.

I see about 3-5 mph of optimism out of all of my vehicles when compared to GPS.

I'd trust the GPS when comparing the speedo to the GPS, when doing a test.

Run down a straight and level highway at a constant indicated speed on the speedo, and the GPS will tell you your true speed.
Do this at several different common speed limits and you'll know how far off your speedometer is.
Using the GPS to replace the speedometer is not exactly as good for reasons already mentioned above.  Its instantaneous readings have some potential for large errors for one thing.  And to really know you need to know how the GPS chipset is calculating velocity and whether it's doing that via doppler, delta position 2d, delta position 3d, inertial, or some combination.  That isn't easy to find out.

If in doubt, find a measured mile on a highway nearby and use a stopwatch to test both.

GPS has a intentional error in the data. Mine shows I'm going .5 miles per hour sitting at my desk.