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both of those methods listed in this thread are notoriously unreliable. The best way is to just carry with you a GPS unit that says the MPH you are going. Google earth, when calculating distance, does not factor in elevation and its detection of elevation isn't not exactly "accurate". Google earth has no idea if you're climbing a highway onramp, how steep it is or anything of the sort to give you an example. As for the MPH radar detectors, they're not terribly accurate.
I suppose the utmost accurate way to measure mph, distance, etc. is to make your own speedometer, measure out the wheel, tire, gearing, etc. everything to the mm, then find an area that is exactly one mile long, mark it with paint, then when maintaining a given speed, with a stop watch, calculate how long it takes to make it across. Then Adjust speedometer accordingly. There is more to it than I've said but the point is, if utmost accuracy is needed, I think it can be figured out how to accomplish it with the most basic instruments.
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