Does the heat play any factor into your cars gas mileage?

Newer vehicles would not be too much of a difference depending on how much you let your vehicle warm up before driving during the winter. It runs less efficiently when cold but that would be offset by using the air conditioner in the summer. My primary vehicle is an 1988 4runner, it increases the idle when it's cold to get to operating temp faster, I don't have a/c. During the winter I get about 10mpg pretty much regardless of whether I let it defrost before driving or not, during the summer it goes up to about 15mpg. My fiance's 2003 Jetta only varies by about 3mpg between summer and winter though.