What Crys said is a good explanation. You could also get a diesel car.
I went from a smaller, gas equipped Mercedes 190E to a larger, turbocharged diesel equipped Mercedes 300D and get 5mpg better with an engine that isn't running optimally...yet. That's my next weekend project.
There are a lot of diesel vehicles that get incredible fuel economy. The diesel Jetta or Golf from Volkswagen come readily to mind but there are many more. Or there are, wait for it...diesel hybrids on the market now that get incredible fuel economy as well. I've only done a small amount of research on it but there is a Volvo diesel hybrid that Volvo claims 100mpg.
I'm a big fan of diesels. They are much more efficient, cheaper to own(over a long period of time), and will last longer if taken care of properly. Just some food for thought