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 like a vehicle treadmill, allowing a drivetrain to operate while the vehicle is stationary. Before these tests, a car is "cold soaked" for 12 hours at temperatures between 68 and 86 Fahrenheit. This treatment, together with the specified driving schedules set by the Environmental Protection Agency (EPA), provide a standard basis for comparing one vehicle to another. Urban driving is simulated using specifically prescribed accelerations, decelerations and periods of idling typical of the stop-and-go driving expected in cities. The highway schedule contains fewer accelerations and decelerations and speeds typical of freeway driving. During these experiments technicians measure both emission levels and fuel economy. Emission levels must meet standards set by the EPA, whereas the Department of Transportation sets the corporate average fuel economy (CAFE) standards, which are reported in miles per gallon (MPG). The measured fuel economy is multiplied by a scale factor to account for the differences between laboratory measurements and real driving conditions. For example, the urban mileage on the sticker of a new automobile sold in the U.S. is actually 10 percent lower than the laboratory measured value and the highway mileage listed is 22 percent less than the laboratory value.
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.
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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 engine¿s 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 vehicle¿s fuel economy, but this is also a problem in warm weather.
Finally, a vehicle¿s aerodynamic drag is proportional to air density. On a 70-degree-F day, the density of the air is 16 percent lower than on a day with temperatures around 0 degrees F. Although this makes little difference in urban driving, it could account for a highway mileage per gallon reduction of 7 percent on the colder day (including a 1.5 percent allowance for improvement in fuel efficiency at the higher engine load).
Considering all these factors, the fuel economy during urban trips of less than 10 minutes, in cold weather with snowy road conditions can easily be 50 percent lower than operation of the same vehicle in warm weather with dry roads.