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Most highway mileage markers are pretty darn accurate. In our state they are marked every 0.1 miles. Start by setting your cruise at 60 mph. And time 1.0 miles using a sweep second hand wristwatch, a handheld stop watch or sport-chrono if you have got it.
If the mile does not take exactly 60 seconds, change the cruise control setting until it does. Now you know what indicated speed is 60 mph. Simply put the car in 3rd, 4th, 5th and 6th at that speed and you will have the RPM at 60 in each of those gears. You can do the math easily to figure out any other RPM per MPH in gear calculation you want.
PS don't get to engrossed in looking at the watch that you lose track of your driving.
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