If you are interested in a Cash for Clunkers trade in, use the calculator below or these charts to see the gas and cost savings of a trade.
The following calculator helps you convert MPG to "gallons per mile" (GPM) so that you can see the gas consumption of different cars. You can select a distance, gas price, and MPG level of your choice. You can compare the GPM of all new 2009 vehicles. Click the image to access the calculator.
The first calculator allows you to convert a car's MPG to GPM for an MPG level and distance of your choice. You will also be asked to enter a gas price to see the cost of driving that car for your chosen distance.
The second and third calculators allow you to compare the GPM of new 2009 vehicles. You will also be asked to enter a gas price to see the cost of driving the cars you are comparing. The MPG figures come from the EPA website fueleconomy.gov (which provides an option for comparing cars based on gallons per 100 miles).
If you are interested in downloading excel sheets and printable tables that convert MPG to GPM, go to the section on this webpage called Tools for calculating GPM from MPG.
Burning one gallon of gas releases about 20 pounds of carbon dioxide (another 20 percent is released in producing gasoline). Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
Update
Chris Shea summarized the argument for "Gallons per Mile" in the 2008 New York Times Magazine "Year in Ideas" issue.
Edmunds describes the shortcomings of MPG and offers a GPM calculator that lets you compare two specific car models in terms of "gallons per 100 miles" at Edmunds.com. (Note: Gallons per 100 miles is summarized as TFC for "True Fuel Consumption".)
If you would like a calculator that converts different combinations of metric (kilometers per liter, litres per 100 km), imperial gallons (as used in Britain), and US measures (such as MPG and gallons per 100 miles), this calculator and this calculator covers it all.