This. At highway speeds, air resistance dominates the energy consumption. It is roughly proportional to air density, which in turn is inversely proportional to

This. At highway speeds, air resistance dominates the energy consumption. It is roughly proportional to air density, which in turn is inversely proportional to temperature. Compare a nice summer day at 300 K, say, to a really cold day at 240 K. That’s a ratio of 5:4, pretty close to the 20% figure mentioned.