In the cold fall and winter months, your car's average miles per gallon can decrease as much as 22 percent. The reason? Colder air is denser, which adds drag, along with a bigger culprit: heat. All the energy used to warm the car plus the defrosters, heaters and seat warmers gobble up gas. To be more fuel-efficient, try to park your car where it's slightly warmer, like a covered garage. If you have a plug-in vehicle, while it's charging, turn on the seat warmers to pre-warm the car so it doesn't drain the battery. And the easiest tip? Remove roof racks: They increase wind resistance in any kind of weather.