Over the last few weeks, we've received hundreds of calls from people who wonder whether they should ditch their old gas-guzzling SUV/truck and buy a newer fuel-efficient vehicle to save money.

Clark usually says that it doesn't make sense to do so. But that's just general advice without looking at the numbers.

If you want to get really granular, Smart Money has a new interactive tool that features 11 variable fields where you can enter your trade-in value, gas mileage, daily driving distance, etc. The tool calculates how long it takes to start getting payback.

You may not like the answer you get. Sometimes the only way to make it work is to buy a used gas-sipper. In this respect, cars from Hyundai and Kia tend to depreciate much quicker than those from Honda and Toyota.

And if you're stumped on your trade-in value, know that Edmunds, Kelley Blue Book and others haven't stayed current with SUV/truck values. So deduct about 25% from whatever they say.