You select the vehicles in question, your location, the local price for gas and the number of miles you drive a month, and the calculator tells you how many months it will take for the fuel savings to equal the money you would need to acquire the new vehicle.

The calculator may actually underestimate how often it makes sense to hang onto a gas guzzler, since it does not account for sales taxes or the immense hassle of having to deal with all of the registration paperwork.

IS A SMALL CAR PRACTICAL? You will be tempted to play with the Edmunds.com calculator by swapping your hulking Chevy Suburban for a tiny Honda Fit or an itty-bitty hybrid of some sort. But let’s get back to reality for a moment. It is nice to fantasize about tripling your fuel economy, but you might have a trailer to tow or perhaps you are larger than average and are not comfortable in small cars.

Say you need to haul three rows of people but still want to save on gas costs. So you trade in your 2005 Ford Expedition for a 2008 Toyota Highlander hybrid with a third row. It will take more than 15 years to break even on that deal, driving 1,500 miles a month, according to the Edmunds.com calculator. The numbers may work better if you get a used car instead.

Tex Pitfield, whose company, the Saraguay Petroleum Corporation, delivers fuel to gas stations, airports and elsewhere, has done the math himself on his 2003 Lincoln Navigator. “I can’t justify trading it in,” he said. “It’s going to cost me more to trade it in than it will to keep driving it.”

WHAT IS YOUR LARGE VEHICLE WORTH? The answer is, probably much less than you think. About 36 million S.U.V.’s were sold in the United States in the last decade, according to Autodata. Plenty of people are blindly putting them up for sale or trading them in right now.