Now to throw another wringer into the discusion. I do several spinning classes a week at the local fitness center. I recently found out the number that everyone thought was milage was actually a number representing the number of revolutions of the wheel. According to the Manufacture it in no way represents any milage figure at all. So check with the company that makes the spinning bike to see if you are actually reading MPH or even KPH.

I can see no real way to corelate the two other than to use "time in the saddle". I enjot both spinning and street riding as they bith have their benefits to me. Enjoy yourself and be safe