•Having bad oxygen sensors and air filters can reduce your gas mileage by up to 20%. The oxygen sensors help keep the proper mixture of air and fuel, and having this off balance can be inefficient. The air filters can get clogged and simply need to be unclogged to fix the problem. Air filters need to be cleaned so that the engine can work at its peak performance.
•Your spark plugs are responsible for sparking combustion in your engine. If they misfire, or are working poorly, this can effect your gas mileage in a negative way.
•Having bad fuel injectors can cause bad mileage. This is because the fuel injectors are responsible for putting fuel into the engine. If there is a leak of some sort, then less fuel will make it to the engine, and then in turn make it run less efficiently.
•Having your air conditioner on causes a lot of gas to be wasted in powering the system. If you can, turn off the air conditioner, and ride with your windows open. At higher speed, close the windows though to reduce drag.
•A common cause of bad mileage is your own driving habits. If you are an aggressive driver, likely you accelerate to quickly. This causes more fuel to be used, and will use up more gas than you need. Same goes for revving your engine. It may sound cool, but it uses up gas for going nowhere.
•Many people waste gas by idling. Idling is when you have your engine on, but you are parked and not moving. Common places where this occurs is when you are trying to warm up your car in the winter, or while you are waiting to pick someone up. Your car really doesn't need more than 30 seconds to warm up the engine. When it comes to waiting for someone, turn your car off or put it into neutral. Since you aren't moving, you might as well save gas and the environment.
•The type of motor oil that you are using can also affect your gas mileage up to 12%. You want to use motor oils that are high in reducing friction. Either use a synthetic oil or the oil that the car's manual recommends.
•The more revolutions per minute you make your engine do, the more gas you will use. The easy way to combat this is to just drive slower. Driving the speed limit of 55 miles per hour is much more fuel efficient than driving at 60, 65, 70, etc miles per hour.
Also, check the air pressure in the tires, make sure the wheel alignment is correct and that the wheels are properly balanced.