I have a generator and I need a wheel that is going to start the generator. How I know the diameter of the wheel that I need to start the engine. The require rpm for start of the generator is 3480-3780. Solution Basically there are many ways to get the dia of wheel but one method is by using the flywheel of corresponding power the flwheel size define the wheel size of the used in generator by using the fluctuation in speed as given.