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BriN
You can get a pretty accurate mph by timing yourself the through a measured mile.

Get up to speed, 60 is a good choice, and maintain that indicated speed when you cross the mile marker. Time the mile. It should be exactly one minute. If it is more, say 66 seconds for instance, then you are going a mile per 1.1 minute, or 60 miles per 1.1 hour. 60/1.1 is 54.4 mph. And you have about a 10% speedometer error.

So if it indicates 70 you are actually going a out 10% less, 70-7 about 63. The error should be linear, 10% at any speed.

One last thing. You can check the odometer the same way. Note the odometer reading as you pass a mile marker, drive ten miles and stop at the tenth mile marker. How many miles did the odometer roll up? 11? Then you also have about a 10% odometer error. You can calculate whatever you get. I just used 10% because the math is easy.