Building a two passenger motor vehicle that gets 125 miles per gallon is easily achievable today, even with a standard gasoline engine.  The concept 3 wheel vehicle shown below would use a 250cc motorcycle engine and tires.  The aerodynamic drag could be 1/3 of a standard car like the BMW shown alongside.  In addition, the tire rolling resistance would be almost 1/3 less  because the overall weight of the carbon composite 3 wheeler is ~600lbs.  Those numbers give  about 3X the fuel mileage of a standard car, even with a normal gasoline engine.  A diesel engine could increase mileage to 155mpg.

This vehicle is not intended for city driving only.  It is meant to cruise at 70mph on interstate highways - it is designed to be as tall and as long as a standard car.  It is not a high performance sports car, it's sole purpose is to achieve maximum fuel mileage with unrestricted range.