Can't you just do it the old fashioned way? Measure how much gas you put in vs miles ridden? I don't know if bikes have "open" tanks anymore, but if they do you should easily be able to fill it up to the same point every time.

BTW - it was always my understanding that most bikes only get up to about 50 MPG or so. My reasoning was that the wind resistance on a bike (and drag) is pretty bad compared to a well designed car.