The government is about to approve a cash for clunkers bill that offers a $4,500 rebate for switching into a fuel efficient car. So which gas-sipper are you going to buy?

Here's a handy guide to figure out which car will be the best for the environment and for gas mileage.

These cars are all small, but they certainly go a ways on a gallon of gas. Every one of them gets more than 40mpg. Or so the manufacturers are telling us.

The Environmental Transport Association, a British charity/lobbying group focused on efficient vehicles, looked at 1300 autos for sale and determined which car was best for the environment. The criteria for judging included fuel efficiency, emissions, power and noise, to name a few.