Why is the fuel economy of a car better in the summer ?

Harold Schock, professor of mechanical engineering and director of the Automotive Research Experiment Station at Michigan State University, explains:

               Temperature and precipitation affect the inner workings of a vehicle and the actions of its driver, both of which have an impact on the mileage. In cold, snowy weather, the fuel economy during trips of less than 10 minutes in urban stop-and-go traffic can easily be 50 percent lower than during operation of the same vehicle in light traffic with warm weather and dry roads.

                Auto components such as electric motors, engines, transmissions and the axles that drive the tires consume more energy at low temperatures, especially during start-up. Oil and other fluids become more viscous as temperatures drop, which means that more work—and thus fuel—is required to overcome friction in the drivetrain components. In addition, the initial rolling resistance of a tire is about 20 percent greater at zero degrees Fahrenheit than it is at 80 degrees F. This rolling resistance decreases as the vehicle starts to move, and in trips of a few miles the temperature rise—and its effect on mileage—is modest.

               The aerodynamic drag acting on a vehicle increases in colder weather as well. Air density is 17 percent lower on a hot, 80- degree day than it is on a cold, zero-degree day. This percentage makes little difference in city driving, but on an open highway the colder temperature reduces mileage by about 7 percent, even taking into account the improvement in fuel efficiency that cars typically experience during highway driving.

                Personal driving habits can also have a major effect on the efficiency slide. In winter, we use heater motors, defrosters and windshield wipers to keep our fingers warm and our sight line clear. We often bring the automobile interior to a comfortable temperature before driving and then keep our engines idling to maintain that temperature when we have to wait in the car.

                 In any season, you can improve your mileage with a few simple steps: Keep tire pressure at the recommended level (lower pressure reduces mileage). Avoid storing excessive weight in the car and driving in heavy stop-and-go traffic. Finally, courteous, careful motorists have lower gas-pump bills than those who employ frequent acceleration and braking.