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Gas mileage for gasoline-powered internal combustion engines tops out at 35 mpg or so for two simple reasons:
1. Gasoline only has a certain amount of energy in it
2. Minimum useability and safety standards require a minimum weight/size
A minimum weight and size means a minimum amount of force needed to accelerate, climb hills, and overcome friction. Combined with a maximum energy content of the fuel gives you a maximum mileage constraint.
The amount of energy in the fuel that can be used is pretty constant if you use a gasoline-powered internal combustion engine (ICE). ICEs are great at utilizing liquid fuels, but at the price of low thermal efficiencies (10-15%). You can increase mileage a bit by sizing the engine and transmission properly, but you're going to top out eventually, and it appears that top is at about 35 mpg.
