• Vehicle gas caps: About 17 percent of the vehicles on the roads have gas caps that are either damaged, loose or are missing altogether, causing 147 million gallons of gas to vaporize every year. 

We discussed in an earlier post that you're unlikely to lose gas to evaporation when you're running low on fuel, but that assumes that it's a closed system --- that your gas cap is present and properly functioning.

• Under-inflated tires: When tires aren’t inflated properly it’s like driving with the parking brake on and can cost a mile or two per gallon. 

• 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 plug 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.

(photo: Jed Kirschbaum/Baltimore Sun)