There is a choppy chart at totalmotorcycle.com under motorcycle fuel economy guide, showing mpgs of various motorcycles. It doesn't have all models of bikes, but there are quite a few listings. The numbers are largely user contributed, and seem on par with what I have observed from friends and family who ride. I wish the list included more vintage stuff, but it's helpful.
Also, the same website has a used bike buyer's guide, which also lists the mpg as part of the specs.
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