text: Gas mileage is usually worse in the cold weather. In the cold weather your tire pressure is lower and causes more friction with the ground. \n\nAlso the type of gasoline you put in your car is different in the summer and winter. Summer fuel is required to have a lower RVP, to mitigate evaporative emissions caused by higher temperatures. It is literally heavier, with more chemical bonds in the fuel molecules... more bonds to break = higher energy content = better MPG.\n\nAnd finally your car also faces more friction driving through cold air because the colder the air is the more dense it is.