Answer:
Sample the speed at regular intervals and compute the average.
Use more samples and/or more regular intervals and/or use a weighted sum that approximates the curve between readings.
A Speedometer is designed to give an approximation of instantaneous speed. It is an average over a short time interval, typically less than a second or two.
If you have only the speedometer as reference, you need to write down its speed reading on several occasions, roughly equally-spaced in time. The average of these readings will be an approximation of your average speed.
To get a better average, shorten the intervals and make them more evenly-spaced. You can also compute various weighted averages of these speed readings.
Step-by-step explanation:
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