[Diesel_Dave]

There are several effects of the cold:

1) Increased rolling resistance. Tires absorb more energy when cold, so it takes more energy to roll. Also, cold lubricants in differentials, transmissions, etc. are also more viscous which hurts FE.

2) Increased drag. Increased air density means more aero drag. A 10 deg C decrease in temp means ~4% denser air, which means 4% more drag.

3) Slower warm-up. The engine looses more heat to the ambient when cold. Also, the cold engine & transmission oil make it more viscous and cause more friction

4) Cooler, more dense intake air. This typically hurts a typical gasoline engine. The reasons are the same as why a warm air intake helps a gasser. Colder air means the throttle closes down more and pumping losses go up. Also fuel doesn't evaporate asnd mix as well, and flame speed goes down. On a diesel, cooler air typically has a slight benefit, at least to a point. Very cold air can delay the combustion and hurt FE.