One of the main reasons cold weather causes vehicles to get less gas mileage is that car engines run most efficiently at a certain operating temperature. In warm conditions, the engine does not take very long to heat up to this optimal operating temperature, while in cold weather it takes much longer. On shorter car trips, the engine will likely never reach the temperature of peak efficiency during cold weather months. On longer trips, the engine will have time to heat up sufficiently, and fuel economy will improve. Another factor that can affect gas mileage is that during winter months many states require that gas stations sell a different blend of gasoline with more oxygen to reduce potential pollution problems associated with more stagnant winter air. This "winter gas" is often reported to burn somewhat less efficiently than regular gas.
Cold introduces a number of factors that affect gas millage due to the fact that colder substances become more dense. Since cold air becomes more dense, it can increase wind drag on vehicles as they drive, lowering fuel efficiency. Cold also increases the viscosity, or thickness, of fluids in the vehicle, such as oil, which causes it to run less efficiently. Since air becomes more dense in the face of cold, the air in the tires also condenses, causing tire pressure to fall. Lower tire pressure results in more friction between the tires and the road, increasing tire wear and decreasing fuel efficiency.
Road conditions are another important factor that affects fuel efficiency during cold months. Cold weather is often accompanied by snow or ice on the ground, which decreases tire traction and subsequently reduces fuel efficiency. Since standing snow and ice can make roads slippery and increase braking distance, drivers tend to go more slowly during the winter driving season. Since most vehicles reach peak efficiency at speeds over 40 mph, driving more slowly results in lower fuel efficiency. Braking more often also results in lower fuel efficiency.
