By John DeCicco, Environmental Defense automotive expert

American cars and light trucks are a huge source of global warming pollution. U.S. autos emit more than 333 million tons of carbon dioxide each year, roughly one-fifth of the nation's total carbon dioxide emissions. In 2004, U.S. cars and light trucks traveled 2.7 trillion miles. That's equal to 10 million trips from the earth to the moon.

Any serious effort to fight global warming must include cutting auto emissions.

The good news? There are a lot of simple steps you can take to cut your car's emissions – and when you cut your emissions you'll also be saving yourself money at the pump.

Before we get to the tips, I can't resist reminding you that your most important leverage comes when you decide to buy a car (or not!). Unless you live in an urban area with good walking, biking and transit options, you're likely to be one of the over 200 million Americans who rely on a car. The best advice is simple: choose the most fuel-efficient vehicle that meets your needs and fits your budget. Choosing your car as if the health of the planet depends on it means giving fuel efficiency greater priority than higher horsepower, extra size and all the "creature comforts" that most auto advertising seduces us to think are oh-so-important. Yahoo!'s Green Ratings are an easy way to compare cars.

But most of the time, of course, we are using the car we have, and there are in fact lots of ways to make the best of it, environmentally speaking. Here are some tips to get you started:

Drive efficiently

• Lighten up! Carrying around an extra 100 pounds in your car reduces your fuel economy by up to two percent. Take with you only what you need and be sure to place luggage inside instead of in the trunk or on the roof to minimize drag and maximize your mileage.
• Take it easy. Nine out of 10 doctors and engineers agree—aggressive driving wastes fuel, not to mention increases stress and accidents! Rapid acceleration and braking reduces gas mileage and can burn an extra 125 gallons of gas per year. Even if the person driving in front of you hasn't seen our tips list, hold your horsepower and keep your cool. And keep your distance, too: tailgating means more fuel-wasting braking, and is one of the worst safety hazards as well.
• Keep it slow. In highway travel, exceeding the speed limit by a mere five mph results in an average fuel economy loss of six percent. You're not on the NASCAR circuit. This is commuting, not racing.
• Don't be an American idle. Idling for more than 10 seconds uses more gas and emits more global warming pollution than restarting your car! Also, the best way to warm up a car in winter months is to drive it. When the temperature is below freezing, give it 30 seconds—that's all you need.
• Hot fun in the summer time. Air conditioning can decrease your fuel efficiency by as much as 12 percent in stop-and-go traffic, so consider cracking the windows. But at high speeds, driving with the windows open can decrease the overall efficiency of the vehicle. At higher speeds, you can use the vents to get a good air flow. On the hottest days keep your AC on low.

Maintain your car

• Don't be tune deaf. Keeping your engine properly tuned can save you up to 165 gallons of gas per year. Checking spark plugs, oxygen sensors, air filters, hoses and belts are a few examples of maintenance that can result in big savings. (Energy Information Administration’s U.S. Retail Gasoline Prices)
• Keep up the pressure. Low tire pressure wastes over two million gallons of gasoline in the United States—every day! Save about a tank of gas a year by keeping your tires properly inflated. And make sure to have your tires correctly aligned to maximize fuel economy.
• Go grease lightning. Thicker than required oil will reduce your gas mileage, because it takes more energy to push through thick oil than it does through thinner oil. Check your owner's manual for the recommended weight, and ask for it specifically when you get your oil changed.

Drive less

• Combine trips. Cutting a 20 mile trip out of your schedule each week can reduce your global warming pollution by more than 1,200 pounds a year and save you over $100 in gas expenses.
• Telecommute once a week. If all commuters worked from home just one day a week, we could save 5.85 billion gallons of oil and cut over 65 million metric tons of carbon dioxide each year.
• Share a ride. Carpool and use public transportation when possible. If you share rides and use other means to get to work, you'll save yourself money, reduce congestion on the roads and cut your global warming pollution.
• Just park it. And keep it there. If you’re going to several stores in the same strip mall, don’t move your car. Walk.
• Gotta wear shades. In summer, park in the shade. Use windshield shades to keep summer heat from baking your car and to help keep frost away in the winter.
• Move your feet. Walk, ride a bike or take the train when your car isn’t needed.
• Avoid the rush. Plan trips during off hours when fewer cars are clogging the roads.