The air density if a function of pressure and temperature: Earth Atmosphere Model - Metric Units The red formula shows the air density is inversely proportional to the ar temperature. Cold air is more dense. The air drag source: The Drag Equation Every vehicle is affected by denser air that increases drag. A rule of thumb, above 45 mph, aerodynamic drive is a primary force which is why every vehicle has lower mileage. But since the drag is proportional to velocity squared, just slow down and the efficiency returns. One thing I forgot to mention is with the colder, winter temperatures, tire air pressure will decrease. So it is time to inflate the tires to your desired pressure. I typically run as maximum sidewall pressure to minimize rolling drag. Bob Wilson
