The problem with the efficiency of a shaft drive on motorcycles has to do with the gearing required to change directions. Motorcycle engine crankshafts and transmission output shafts generally run transversely, as does the axle. The drive shaft runs fore-and-aft. So a bevel gear is used to change the transverse motion of the transmission to the longitudinal motion of the shaft, and another bevel gear changes the shaft direction to the wheel direction. Some motorcycles such as Gold Wings, Moto Guzzis, BMWs and some others have longitudinal crankshafts, so only have one direction change.

Way back when the Wright brothers were designing their Flyer, they were told that chain drive was pretty lossy. They did a good bit of experimenting, and as I recall reading they came up with an efficiency of about 95% for chain drive. They went on to successfully drive the propellers of the Flyer with chains.

Okay, I think I understand what ya'll are saying then. But ya'll are referring to the typical v-twin where the cylinders are in line with the bike frame correct? If so, then in the case of my brother's GL500 where it is a transverse v-twin, would the shaft be more efficient than a belt or chain? Just curious not trying to spark an argument or anything.

 Drive shaft also loses energy trying to lift the back end of the motorcycle.

 Instead of the back going down when accelerating It is lifted up,Drive  one and you will see.

 I think instead of the force coming from the front of the bike its coming from down low at the rear of the bike.

I read in v-twin mag last year that the chain lose 5% belt 8% and 15% for the shaft power to the rear wheel. Thunder manufacturing recently came out with a chain conversion for the meanstreak why cause the shaft looses to much power.

From memmory, bevel drives are inherently inefficient.
The teeth are trying to slip by each other transversely like a worm drive.
And with bevel drives you have something not present in others... thrust bearings.