Motorcycles that get 50 or 60 miles-per-gallon are fairly commonplace these days. A few machines do even better (the new Kawasaki Ninja 250, for example). 100 mpg is harder to come by, but that could change with diesel.
The company that has been making diesel versions of the Kawasaki KLR650 for the military for several years is planning a street-legal version capable of 100 mpg. This company is Hayes Diversified Technologies located in California. Take a look at its web site.
If a relatively small manufacturer largely unconcerned with emissions (as a military supplier) can create a street legal 650 capable of 100 mpg, what do you think the major OEMs could do? Send us your thoughts. We want to hear from the automotive, motorcycle and diesel engineers out there.