I'm probably repeating myself, but jesus, a lot of small cars will do 60mpg + if driven carefully, and you are talking about a load of mass, rolling resistance and air resistance. A middleweight bike, with a fraction of all of these, struggles to match the car, never mind beat it. How hard can it be to make a 500cc bike that would do 100mpg in normal use? My guess is that bikes tend to be leisure (or part leisure) tools, and people tend to subconsciously compare the fuel consumption to a car. Plus, the manufacturers have spent nothing like the money on engine development. There has to be a market for something like the old Kawasaki GT550 - shaft drive and no hassle - but with much better economy.