When I was looking for a new motorcycle, I tried to find the highest mileage scooter that was capable of not getting run over on the highway. You would think that mileage ratings for motorcycles would be more transparent, but there is no government mile per gallon standard that manufactures are required to publish with the sale of the motorcycle so it becomes a guessing game based on internet searching.
Ironically, most motorcycles with a usable/flexilble engine size really don't get very good mileage and many with huge over 1,000cc displacements rate below my Civic Hybrid which has about the same size gasoline engine (1,200cc). I was replacing a 919cc in-line 4cyl Honda that got in the 45-50 mpg range and before that a carburated 4cyl Honda that got about the same.
For most people buying a motorcycle, it is more an emotional experience of coolness and/or chrome and that is how they are marketed. It's about the ride and the "lifestyle" more than a statement of responsible fuel use.
It is assumed that any motorcycle will get great mileage and if you compare it to most 
four wheeled vehicles on the road 45-50 mpg is probably double. However, from the perspective of a car owner who normally gets 45-50 mpg in my car, I expect more from a motorcycle.
