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How to check actual RPMs of a scooter motor?

I want to be able to check the actual (not rated) RPMs of my scooter motors to see what (if any) speedup I might get from overvolting an E300S 250 watt motor.  I am not even 100% sure it is 250 watts to begin with but I am assuming.  So the idea I had would be to measure a long straight strip of pavement using my car (such as 1/4 mile), and see how long the scooter takes to make that 1/4 mile trip using a flying start (getting the scooter up to full speed before the start line).  The I could just "back calculate" the motor RPMs, since I know the rear tire diameter and the # of sprocket teeth.

My other idea would be to mark the rear tire with something like a white dot, and with the scooter up on a milk crate (or cinder block), tape the throttle open, take a movie of the rear wheel for something like 15 seconds, then go back and play the movie in slow motion so I can count the number of wheel rotations. I can probably get away with that cuz my scooter has low "gearing" but I suspect this method might fail for a scooter with a much higher top speed.

Any other ideas I am missing to check this?  I think both methods are useful cuz one is at full load and one is at a greatly reduced load.  It would be interesting to see what (if any) difference there is between just those 2 scenarios, before trying any overvolting, then repeat both methods with the 6V (25%) overvolt (24V to 30V).

I tried using method 1 which is to mark off a known distance (using my car's odometer), and then driving that distance on the scooter using the flying start method.  I went 0.3 miles in 2 minutes (2 way average) on my E300S so that means it is "geared" (sprocketed) for 9 MPH max but since I have 9:89 sprockets, that means the motor is spinning at about 3000 RPMs, not 2650 like I think the motor is rated for.  That means as the scooter is "geared" down, you don't lose as much top speed as the numbers might make you think.  My scooter is only supposed to be going about 8 MPH top speed according to the "numbers" but the load is so light that it seems to motor spins out higher.  When I add the 4 extra volts to the 24V battery bank (for 28V total), if the motor RPMs are proportional to the input voltage, I should get about 3500 RPMs max which should be good for 10.5 MPH top speed.  That should be good for flat roads and hill climbing without having to upgrade the motor.  The extra 4 volts should make the 250W motor perform more like a 350W motor.  I didn't yet try the slo-mo movie technique but I might soon.

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