• Worn spark plugs - A vehicle can have either four, six or eight spark plugs, which fire as many as 3 million times every 1,000 miles, resulting in a lot of heat and electrical and chemical erosion. A dirty spark plus causes misfiring, which wastes fuel. Spark plugs need to be replaced regularly.

• Dirty air filters - An air filter that is clogged with dirt, dust and bugs chokes off the air and creates a "rich" mixture - too much gas being burned for the amount of air, which wastes gas and causes the engine to lose power. Replacing a clogged air filter can improve gas mileage by as much as 10 percent, saving about 15 cents a gallon.