One day, cars may run as quietly as hummingbirds, their exhaust pipes emitting nothing more than air and water. But right now, transportation accounts for a third of carbon-dioxide emissions in the United States -- and a whopping 60 percent of that, according to the EPA, comes from private vehicles. As prices rise at the pumps, there's never been a better time to rethink your driving habits or invest in a more fuel-efficient ride.
Perhaps the best way to lower your carbon footprint is to limit your car time: Group errands geographically, carpool, and take public transportation whenever possible. There are also a number of ways to make your car more fuel-efficient. First, get a tune-up. A clogged air filter, dirty spark plugs, worn-out belts and fans, and a history of neglected oil changes can cost you upwards of 165 gallons of gas a year.
Keeping your tires pumped to the recommended pounds per square inch (psi) can save you an entire tank in a year, according to the advocacy group Environmental Defense Fund. The EPA and Department of Energy report that traveling light and emptying the trunk of nonessential items also improves fuel economy.
Driving erratically -- jackrabbit starts, quick braking, and speeding (even just a little more than 60 miles per hour) -- can reduce gas mileage by 33 percent. On highways, stay at the speed limit and use cruise control to keep from slowing down and speeding up. Avoid idling; turn off the engine when you can. Air-conditioning also chips away at fuel efficiency (up to 2.2 percent).
If you're in the market to buy a used car, aim to improve your fuel economy. Most Americans buy used gas- or diesel-run cars (although some used hybrids are now on the market). "Buy the highest-fuel-economy car or truck that meets your needs," says David Friedman, research director for the Clean Vehicles Program at the Union of Concerned Scientists.
Friedman advises getting the type of vehicle you need on a daily basis, rather than going for a big truck or SUV that you may need only once or twice a year. Unless you haul big objects often, it's wiser to rent a big rig when you require one. Visit fueleconomy.gov for mileage estimates for vehicles made between 1985 and 2008.
If you're purchasing a new vehicle, you're in an even better position to lower your carbon footprint. The EPA measures new cars' emission levels and fuel economy, giving them a SmartWay grade -- similar to the Energy Star grade for appliances.
See how different cars score at epa.gov/greenvehicles. Hybrids -- which run on both electric motors and gas engines -- undoubtedly get the best fuel economy (sometimes up to 45 miles per gallon) and produce the lowest tailpipe emissions. Low-emission diesels have long been the norm in Europe but have only recently made their way to the United States. The new Mercedes E320 Bluetec sedan uses an exhaust-treatment system that removes most nitrogen-oxide emissions. Volkswagen plans to release a similar diesel this year, and Audi, BMW, Honda, and Nissan are developing comparable cars.
Diesels also tend to get excellent mileage (up to 40 percent better than conventional gas engines) and can be converted to biodiesel, a renewable fuel made of plant oils and animal fats. Biodiesel is usually blended with petroleum-based diesel; the most common mixture is known as B20, which is 20 percent biodiesel and 80 percent petroleum. Flex-fuel cars run on ethanol, which is usually made of corn. There are some downsides to biofuels, however: Filling stations are few and far between, and recent research has shown that producing biofuels may create more harm than good when virgin grasslands and rainforests are converted into croplands to grow the feedstock for those fuels.