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Cars Get Better Mileage When...

Assuming all things are equal, does a car get better mileage if the roadway is wet or dry, the air is very humid or dry, the altitude is up say a mile or at sea level, and the temperature is very cold or very hot?

ANSWER: According to members of the Society of Automotive Engineers a car gets better mileage:

– when the road is dry. This is because there is better traction and the power is transferred to the road more efficiently.

– in humid conditions. This is because there is less need to throttle the engine. Throttling is a way of controlling the speed of an internal combustion engine, but it takes some of the engine's power.

An internal combustion engine is a cylinder in which air and gasoline are mixed, compressed by a piston and ignited. In the first step of the four-stroke combustion cycle used by most cars, a valve opens to take in air and gasoline as the piston moves downward. Throttling closes the intake valve for part of the time the piston is moving downward, forcing the piston to pull against a partial vacuum, which wastes energy.

For a particular power output, the engine needs a constant amount of oxygen to burn the required amount of fuel. When there are more water molecules in the air, some of the oxygen molecules are displaced. Therefore, in humid conditions, the engine must take in a greater volume of air to get the same amount of oxygen. The intake valve can remain open longer, and less work is required to pump gases through the engine.

– at high altitude. This is because there is less drag on a vehicle in thinner air. It also takes less effort to expel the exhaust because the atmospheric pressure “pushing back” on the engine is lower. There is also less throttling because a larger volume of air must be taken in to get enough oxygen to burn the same amount of fuel.

– when it is very hot (assuming the air conditioning is off). This is because the air density is lower, so there is less need to throttle the engine.

You may not be able to change where you drive or the weather conditions, but making certain that your tires are properly inflated is an excellent way to improve mileage. Inflating more than manufacturers' recommendations can reduce traction, but inflating too little can reduce the size of your wallet. With too little air, the tires flatten out, resulting in increased rolling friction, which acts to slow down the motion of the wheel and decrease gas mileage.

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