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Condition	Effect	MPG Penalty up to
Under inflated tires	Increase rolling resistance	1-2mpg
Dirty air filter	Causes excessively rich fuel/air mixture	2.0mpg
Worn spark plugs	Cause inefficient combustion, wasted fuel	2.0mpg
Worn O2 sensor	Unable to detect and adjust air/fuel mixture	3mpg
Dirty or substandard engine oil	Increases internal engine friction	0.4mpg
Loose gas cap	Allows fuel to evaporate	2.0mpg
Potential loss in fuel economy if all of the above were neglected		11.4mpg

The Car Care Council offers these fuel-saving tips:

1. 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.
2. 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.
3. 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 as recommended by the manufacturer.
4. 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 20 cents a gallon.

Fuel-saving driving tips include:

• Don't be an aggressive driver -- Aggressive driving can lower gas mileage by as much as 33 percent on the highway and 5 percent on city streets, which results in 10 to 66 cents per gallon.
• Avoid excessive idling -- Sitting idle gets zero miles per gallon. Letting the vehicle warm up for one to two minutes is sufficient.
• Observe the speed limit -- Gas mileage decreases rapidly at speeds above 60 mph. Each mpg driven over 60 will result in an additional 10 cents per gallon. To maintain a constant speed on the highway, cruise control is recommended.