>>2947274
Youtube and google how to fix whatever breaks.

>>2947271
Why make it that complicated? Put a piece of masking tape on the belt and use the stopwatch on your phone.

Otherwise tachometers aren’t expensive. Put the piece of tape on a pulley that drives the belt, point the tach at it, do the math with RPM of pulley, size of pulley, length of belt and you will get a ratio of RPM of the pulley to revolutions of the belt.