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I agree with Eberhard in regards to speedo accuracy, regardless of what has been observed by the owner at 60 mph, etc.
This we can compare with a torque wrench:
If you want a torque wrench to be completely accurate, the best route is to get a torque wrench that was designed for only one particular torque, ie non-adjustable. The disadvantage is that we all need torque wrenches to work at a variety of torque settings. Good quality variable torque wrenches will never be as accurate as a purpose-built non-adjustable torque wrenches.
With speedometers, you can get the most accurate reading if the speedometer was calibrated at one speed. But that would be almost as useful as a non-adjustable torque wrench (although non-adjustable torque wrenches are used on assembly lines).
The speedometer will have variations in it because it is meant to be somewhat accurate at all speeds, but usually is dead-on accurate in one or two speeds. Typically the speedo will be the most inaccurate at the highest speeds. It doesn't surprise me that the speedo would be accurate in the measured-mile test.
You could maybe calculate what the measured mile time would be for the higher speeds and do a test run at that speed to test your speedo.
Gilly
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