This was an unknown while I was riding(I never counted), while I was using the android app I used the known data I had from the cheap meter: Max speed (15.2), and from the hardware specs: wheel(20x1.75),crank size(170mm),chainring(42t),cog(20t) and with this entered in the app the slider for cadence I moved until my speed showed 15.2 and this is where I got cadence. I'm not sure that worked like that but when I changed the cog to 16t from 20t I got 19.1 mph.