Engine RPM

I have been doing the opposite due to the wireless bicycle computer not working when the engine is running. (I think the tach wire might be a radio aerial, but I'm not so sure I'm ready to give up the tach for the speedo)

I use the calculator at http://www.bikecalc.com/speed_at_cadence and preferably an actual rollout measurement, but the drop down list of tyre sizes should be close enough. I enter sprocket sizes halved for the rear and times five for the front (I enter 21t for the rear and 50t front) and set cadence as 100 so that the figure for speed @100rpm pedalling cadence is actually 1000rpm at the engine output sprocket, or 4100rpm. Dividing that speed by 4.1 gives the speed per 1000rpm. The speed figure given by bikecalc is only to two decimal places so I suppose this is a slightly rounded figure but it's close enough for me. :)

The calculator does cadence needed for a target speed too, so I'm sure you can reverse my arithmetic. :)
http://www.bikecalc.com/cadence_at_speed
 
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