Faced with longer waits at airports, you might be tempted to hop in the car and drive over the river and through the woods to Grand­mother’s house, or wherever your travel plans take you. But what about gas prices? Here’s how to keep costs down.

Drive smoothly. In our tests with a Toyota Camry, frequent starts and stops reduced mileage by 2 to 3 mpg.

Reduce aerodynamic drag. At highway speeds, more than half of the engine power goes to overcoming drag. Driving with a car-top carrier cut 6 mpg from our family sedan.

Slow down. Drag rises with speed. Slowing from 75 to 55 mph boosted gas mile­age 33 percent in our sedan and in a large SUV.

Turn it off. Shut off the engine if you’ll be idling for more than 30 seconds.

Use regular gas when possible. Think twice about premium even if the manual suggests it “for optimum performance.” But if your car pings or knocks, use premium.

When possible, walk, bike, or “chain.” Half of all car trips are less than 6 miles. String together your pickups of pumpkin pie, cranberries, and other fixings into one long trip instead of many short ones. Driving with a warm engine saves gas and limits pollution and engine wear.

If you’re buying a new car, consider a high-rated fuel sipper such as a Toyota Prius (44 mpg overall in CR’s tests), Honda Civic Hybrid (37 mpg), or Toyota Camry Hybrid (34 mpg). High-rated, thrifty nonhybrids include the manual Honda Fit (34 mpg) and Honda Civic (31 mpg). For an SUV, consider the Toyota RAV4 4-cylinder (23 mpg).

Don’t use “gas savers.” In past tests, Fuel Genie, Platinum Gas Saver, and Tornado did not boost fuel economy.

Don’t fret about the A/C. If you’re heading to a warm climate, cool off. Air conditioning uses about 1 mpg, but safety (and comfort) increase with use. Opening windows made no significant difference in our gas mileage.