If your car has a blanket of snow or a layer of ice covering it, make sure you clean it all off before beginning to drive. The snow or ice will increase the air resistance of your car and force the engine to burn more gas fighting it. This is especially true if you quickly take your car up to highway speeds while it still bears a lot of snow. At high speeds the air resistance is much greater. On top of that, in the winter the cold air is denser, which also increases air resistance. If you can, park your vehicle inside or under some shelter so it does not accumulate snow or ice. Another advantage to parking inside is that the vehicle will probably be a little warmer and so will run a little bit more efficiently. There is the safety factor too ... driving along the highway with your personal snow cloud blowing off the roof of your vehicle can blind drivers behind you, thus precipitating an accident.