Gas prices might have decreased recently, but a fill-up can still put a hefty dent in your wallet, especially if you are driving an older, less fuel-efficient car or a truck, sport utility vehicle or crossover vehicle whose fuel economy is diminished because of its size. Those car ads touting 30 mpg or higher can be very tempting, but in most cases trading in your current car just to get more mileage doesn't make financial sense. Here's how to determine if you should trade in your vehicle for a more fuel-efficient one.

1. Calculate how many miles you drive. The typical American drives an average of 15,000 miles per year, but your situation may be radically different. The more miles you drive, the larger the savings you'll see from a fuel-efficient car over your current model. Measure your round-trip mileage for work, and multiply it by the number of days you work in a year. Next, add extra miles for your typical, nonwork driving activities. Don't forget to add additional mileage for your annual summer vacation (if you drive) or any other road trips you may take.

2. Assess how expensive gas is in your area. Gasoline prices vary dramatically depending on where you live. On national average, there's about a 50-cent difference from the cheapest parts of the country to the most expensive. The more expensive the gas is in your area, the more you'll save by driving a fuel-efficient car. Assess the average price you pay per gallon of gas and use that number in your calculations. If you'd like more specific numbers, check out the "Weekly U.S. Retail Gasoline Prices" or "U.S. Retail Gasoline Historical Prices" Web pages from the Energy Information Administration website.

3. Determine the fuel economy of your car and the one you may buy. Next, compare the miles-per-gallon rating of your car and any possible replacement car. For your car, you can use the rating from the Environmental Protection Agency or the number from your car's trip computer. The most accurate option is to start with a full tank and, at the next fill-up, divide the miles driven by the gallons of gas used to get your mileage number.

For cars you are considering, use the EPA rating, which is located on the cars' window stickers or can be found at FuelEconomy.gov. If you are considering purchasing an older car, the site is especially helpful because it features new, more accurate numbers for 2007 model year cars and older. The EPA changed the way it calculates its miles-per-gallon ratings for the 2008 model year to put them more in line with real-world estimates.

4. Calculate your savings. Once you've calculated your personal mileage, the typical gasoline prices for your area and the fuel economy of your current car and the cars you are considering, you can determine your savings if you were to buy a more fuel-efficient car. Calculating your potential savings is easy with the U.S. Department of Energy's calculator at FuelEconomy.gov. Simply select the button that calculates costs using "separate city and highway mpg," and enter your numbers for your current car and the cars you are considering to see the cost difference.

The likelihood is those numbers are surprisingly low. But even if you think it's worth it, assess other costs of ownership, such as monthly payments, insurance, maintenance and repairs before deciding whether to take the plunge and buy a new car.