Kayla that is a question we are asked frequently and there are two major reasons for the drop in fuel economy during the winter season. First, the colder the outside air is the more densely packed the air molecules are. Internal combustion engines run on a mixture of air and fuel with the ideal ratio of 14 to one. In dense air, it takes more fuel to arrive at that ideal ratio. Even though the difference is minute, it can lead to slightly lower fuel economy when driving in cold air conditions. Secondly, refineries are required to change the blend of fuel for winter months to help reduce harmful air quality emissions and meet EPA standards. The winter additives change the Reid Vapor Pressure (RVP) of the fuel mix. RVP is a measurement of the volatility of the fuel; summer blends have low RVP while winter blends have a higher RVP. According to the EPA summer blends have 1.7% more energy; the low volatility provides for a better, more complete fuel burn. When fuel burns completely there is less waste, lower emissions, and more power is produced from the burn thus; the vehicle runs more efficiently. That is the reason vehicles get better fuel economy during spring and summer months.
