On 5/7/2013 11:01 AM, David Bjorkman wrote:
> Before you go blaming the speedo (which is well documented as being
> off) and take the GPS as the final word. do not forget that there is an
> error in accuracy in your GPS also.  There should be a place you can
> check which satellites are being used by your GPS and the expected
> accuracy.  One may be surprised to find that that error can be more than
> just a few feet.

Modern GPS units don't compute speed based on how long it takes to move
between points, so are not subject to errors in location.

They measure the Doppler shifts of the carriers.

"Generally most manufacturers claim a 0.1MPH tolerance with speed
readings, however will go as far to say a 0.5MPH inaccuracy is possible,
to make allowances for any satellite signal variations beyond their
control." - http://gpssystems.net/accurate-gpsbased-speedometer/