Theres a ton of wasteful energy in the 4 stroke internal combustion engine, so theres probably a theoretical limit to just how much gas mileage you can expect to get out of that design.
http://en.wikipedia.org/wiki/Four-stroke_engine
Otto engines are about 35% efficient – in other words, 35% of the energy generated by combustion is converted into useful rotational energy at the output shaft of the engine, while the remainder appears as waste heat
Modern engines are often intentionally built to be slightly less efficient than they could otherwise be. This is necessary for emission controls such as exhaust gas recirculation and catalytic converters that reduce smog and other atmospheric pollutants.