The average gasoline internal combustion engine is only 20% efficient. That is to say, only 20% of the energy present in the fuel-air mix at ignition is reclaimed as mechanical energy by the engine; the rest is lost as heat. A further 90% of the energy harnessed by the engine is used to keep the engine itself running; pumps, belts, fans, and transmissions all take energy to run. That means only 2% of the energy present in the gas tank makes it to spinning the wheels. And that's with the air off! So you can see, there's a lot of room for improvement.
Turbo Diesel engines are half again as efficient than a gasoline engine of the same weight. If every gas engine were swapped for a Deisel engine, the US could stop oil imports from all other countries except Canada. And that's just going from 2% efficiency to ~3%. If you were to add gas turbines in the exhaust system to capture some of the waste heat, essentially making a multiple-expansion engine, you could easily tripple or quadruple current mileages. Combine such an engine system with and electric powertrain, like the Chevy Volt, and regenerative braking and you could have a 100MPG car within the decade. All using existant technology. So don't tell me a 54MPG fleet average is unattainable. If it was, the car makers themselves wouldn't have agreed to it.