The measuring device for rotational speed is a tachometer. But they are expensive and not easy to find.
Its cheap and easy to make one using a bicycle speedometer (cyclocomputer). In fact, the only thing you need is a functional cyclocomputer that reads speed in miles per hour. You won't damage it, so you can even 'borrow' one from your bike, or add it to your bike once your done!
You will only need 1 thing:
A cyclocomputer that reads in MPH and that lets you enter a wheel size in millimeters. Almost all of them do.
You will have to mount a magnet to your spinning wheel, engine shaft or what not.
And you will have to mount the speed sensor near the path of the spinning magnet.
Step 1: Theory and numbers - skip if you don't care!
When you first install a cyclocomputer on a bicycle, you have to enter the circumference of the wheel in millimeters. With that information, it can compute how far and how fast you've gone with every turn of the wheel.
In this instructable, we are going to enter a special number into the cyclocomputer for the wheel circumference that will trick it to report speed in rpm.
If the cyclocomputer is reporting speed in mph, enter 268 mm for the wheel circumference.
If the cyclocomputer is reporting speed in kph, enter 167 mm for the wheel circumference.
Here is the calculation...
1 mph = 1.61 kph.
1.61 kph / 60 minutes in an hour = .026833 kilometers per minute
.026833 kpm* 1,000,000 millimeters per kilometer = 26,833 millimeters per minute
26,833 mmpm / 100 (scale factor) = 268 mm
268 / 1.61 mph-kph conversion = 167 mm
Note: some cyclocomputers might not be able to accept a number below 200, so use the 268 mm number, reporting in mph might be better.
If you ride, you may know that some cyclocomputers also record cadence, which is how fast you are pedaling - and that number is in RPM! So the cyclocomputer already is a tachometer! But they can only read to a maximum of 199 RPM, which is way faster than a person can pedal anyway. I guess if you needed to measure a really slow speed it could work, but this instructable lets you measure a much wider range of speeds.