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In addition to all the temperature dependent factors of efficiency and seasonal fuel blends, a large factor is simple air density increase. Aerodynamic drag force (F) = 1/2 Cd * rho * V^2 * Aref . With all other vehicle factors unchanged the variable from summer to winter is air density rho. At 70F and 50% RH air density is 0.07373 lb/ft^3. At 20F and 30% RH air density is 0.0816 lb/ft^3. The density difference from summer to winter in this example is (0.0816-0.07373)/0.07373 = 10.7%. This is also the increase in aero drag force over that temperature swing. It is not unusual for Class 8 fleet operators to report a 10% fuel economy reduction from summer to winter.
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