Americans drive about 3 trillion miles every year, according to the Environmental Protection Agency. More than 70 percent of United States oil use is for transportation, which means doing small things to help reduce your gas usage can help save precious natural resources and money in your wallet, too. Take a look at your car's owner manual: Are you using the grade of motor oil it recommends? Using the wrong kind -- or not knowing and trusting the auto shop to choose -- can lower your car's gas mileage up to 2 percent. Changing to the proper oil grade won't cost more, and it will save you money at the pump.