Keep in mind that changing sprocket size will change the indicated MPG because the odometer readings will change relative to actual miles traveled. For instance, I changed from a 46 rear to a 43. If my MPG after the change calculates as 50, it is really about 53.5, because the odometer is turning 7% less than before for the same number of actual miles. As I understand it, the odometer is relatively accurate with original gearing; I have found my odometer now to be approximately 7% short of actual mileage. Surprisingly, now my speedometer is indicating a more correct speed.
This makes it even more difficult to determine how sprocket sizes affect MPG. I prefer the more accurate speedo, the lower RPM at actual speed, and broader range of speed in each gear which is the result of lowering the overall ratio. I suspect that all things being equal, my MPG has probably improved. But as I said, itís hard to prove.
The biggest thing that affects MPG is your right hand.