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Or you can just go find a nice flat stretch of interstate highway, set your speed at 60 mph, when you pass a mile marker look at the second hand on your watch, when you get to the next mile marker, your second hand should be in the exact same spot. You may want to check 3 or 4 consecutive miles, for a more accurate test. 60 mph = 60 second miles
If you divide 3600 (which is the number of seconds in an hour), by how many seconds it takes you to cover a mile, you get your actual speed in mph. Example: it takes you 63 sec to go 1 mile.....3600 divided by 63 = 57.14 mph. Or a 56 second mile...3600 divided by 56 = 64.28 mph.
This is how this old trucker used to do it before I retired.....but back then we didn't have gps, or smart phones
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