Why is winter fuel economy lower?

Cold weather affects your vehicle in more ways than you might expect:
•Engine and transmission friction increases in cold temperatures due to cold engine oil and other drive-line fluids.
•It takes longer for your engine to reach its most fuel-efficient temperature. This affects shorter trips more, since your car spends more of your trip at less-than-optimal temperatures.
•Heated seats, window defrosters, and heater fans use additional power.
•Warming up your vehicle before you start your trip lowers your fuel economy—idling gets 0 miles per gallon.
•Colder air is denser, increasing aerodynamic drag on your vehicle, especially at highway speeds.
•Tire pressure decreases in colder temperatures, increasing rolling resistance.
•Winter grades of gasoline can have slightly less energy per gallon than summer blends.
•Battery performance decreases in cold weather, making it harder for your alternator to keep your battery charged. This also affects the performance of the regenerative braking system on hybrids.

In severe winter weather, your mpg can drop even further.
•Icy or snow-covered roads decrease your tires' grip on the road, wasting energy.
•Safe driving speeds on slick roads can be much lower than normal, further reducing fuel economy, especially at speeds below 30 to 40 mph.
•Using four-wheel drive uses more fuel.