The average mid west winter months daily temperature of 25°F (-4°C) is responsible for a 13% fuel mileage penalty compared to summer time conditions of 70°F (21°C) or higher. Cold air is denser and increases the aerodynamic drag on the tractor-trailer. High winds, terrain, and snow-covered roads can also change fuel economy by an additional 13% compared to a calm day and well-maintained roads.