I just downloaded the app but haven't tried it yet. It won't read the rpms directly, it emits a strobe flash at an adjustable rate. Looks like of you put a mark on the flywheel then adjusted the flash till the mark appears to stop, the readout should be your speed. If the readout is accurate, which the instructions say it might not be due to differences in different phones. You would have to test it on something with a known speed first.