When I heard about this tool, I was all excited, sure I had found a reason for my husband and I to upgrade our cars. Not so much. But perhaps you will have better luck.

Edmunds.com has released a new tool called the Gas Mileage Savings Calculator (hereafter referred to as the GMSC.) The URL is http://www.edmunds.com/calculators/gas-guzzler.html . Here's how it works. Click on "Select a Vehicle" and Edmund's will walk you through the process of getting the make and model information of the car you want to buy. After you select the vehicle Edmund's will give you information like the vehicle's cost and MPG. (The cost is going to be approximate, of course.)

After that you choose the car you want to trade in. (This only goes back to 1990; you can't get information on trading in your Ford Pinto.) Edmunds.com will give you an approximate trade-in value (No matter what car I looked at I got SOME kind of trade-in value. Frankly, a few times I was expecting to get $HAHAHA instead of a dollar value.)

Finally, you're asked to enter the number of miles you drive a month and the cost of gas per gallon. With all that information, The GMSC will note the MPG of your current car and your potential new car, how much a month you'll save on gas if you upgrade, and how many months it'll take before you've broken even and will start saving money on gas.

I gravitate toward small, fuel-efficient cars anyway, so I couldn't find a scenario using this tool that made it sensible for me to upgrade my car. But trying different combinations of cars found some interesting results. For example, say I'm the proud owner of a 1992 Chevy van and I want a 2008 Honda Civic. According to the GMSC, the Civic will cost about $16,000 after calculating trade-in. The Civic gets 30 MPG, and my van gets 17 MPG.

The mean work travel time for commuters in Wake County, according to the US Census Bureau, is 24.7 minutes. We'll call it 25 minutes. To make it simple, let's say that the average travel speed to work is 30 miles an hour, making this a 12.5 mile commute, or 25 miles a day. 25 miles times 30 days in a month (the weekend mil age is for going to the store etc.) is 750 miles a month I'm driving my groovy van around.

According to the GMSC I will be saving $76.47 a month if I upgraded my van to the new Honda Civic (this is assuming gas is $4 a gallon). I might not consider that savings enough to make the upgrade. On the other hand, say my driving is twice that -- 1500 miles a month. And say gas goes to $5 a gallon. Suddenly I'm saving $191.18 a month. With that kind of savings I could be making half a new car payment with the money I'd save on gas.

Wanting to save money and wanting to conserve energy -- both admirable goals! The GMSC gives you a way to play with different new car ideas and gas pricing scenarios without having to go out and make test drives. But you may not get the rationalization you're looking for to turn in your good old car...