Harold Schock, professor of mechanical engineering and the director of the Automotive Research Experiment Station at Michigan State University, explains.
Every new car sold in the U.S. has a manufacturer-provided fuel economy sticker describing mileage for urban and highway driving. This fuel economy is measured by operating the vehicle on a chassis dynamometer using a repeatable driving schedule. A chassis dynamometer acts
In contrast to the precisely controlled experiments described above, a typical driver does not operate under ideal conditions. The actual mileage (MPG) that a given vehicle will deliver depends on three general factors: driving style, road conditions and the additional energy consumed when the outside temperature decreases. For example, aggressive drivers will obtain fuel economy values that are lower than those achieved by more cautious drivers. Vehicle fuel economy can be severely diminished if additional work is required to move a vehicle through snow or water on a highway or if excessive weight is stored in the car. Tire slippage can occur on wet or icy highways, which wastes energy and decreases fuel economy. In addition, in cold climates many people bring the interiors to a comfortable temperature before driving and keep their engines idling during prolonged waiting periods to maintain that temperature. Excessive stop and go driving in heavy traffic, use of heater motors, windshield wipers and defrosting devices all cause additional fuel consumption and reduce fuel economy. It is difficult to estimate exact percentages of fuel economy reduction for these factors as they vary considerably between drivers and different road conditions, but any time a car is warming up or not moving with the engine running, the fuel economy is 0 MPG.
Auto components such as electric motors, engines, transmissions and tires consume more energy at low temperatures, especially during startup. The viscosity of the oil and other fluids increases with decreasing temperature, which means that more work and more fuel is required to overcome friction in the engine, transmission and other drivetrain components. If the outside temperature is significantly below the ambient temperature at which the EPA prescribed tests were conducted and trips are short, the engines coolant system never reaches normal operating temperature and more fuel is utilized. Additionally, the amount of drag between tires and the road is about 20 percent greater at 0 degrees F than it is at 80 degrees F. Operating tires at lower than recommended operating pressure further degrades a vehicles fuel economy, but this is also a problem in warm weather.
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