-Driving with underinflated tires is similar to driving with your parking break on. To save an extra 1-2 miles a gallon, make sure your tires are properly inflated. -One misfiring spark plug can be enough to cost you up to 3 miles a gallon in wasted gas. Replace spark plugs regularly. -To save up to 10% of your gas mileage, replace your air filters when they get dirty. -Make sure to tighten your gas caps. -Dirty oil costs you 1 mile per gallon. Change your oil frequently. -Check for a slipping automatic transmission, as it can cost you a mile a gallon. -A faulty cooling system thermostat can waste up to 2 miles a gallon. -Drive slower! You will get better mileage driving 50 miles per hour than 80 miles per hour. -Keep excess weight, like sand bags, out of your trunk. -Slamming down on the accelerator or brake pedals and speeding will affect your mileage. The less aggressive you drive, the more gas you'll save. -Use a high gear on the freeway. -Only run your air conditioning if you really need it. Although, if driving down the freeway, having your windows rolled down will create a drag in your car, burning more gas. -Don't use snow tires in good weather.