125 Miles per Gallon
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.