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

Your car's engine has a temperature sensor on the air intake, because it needs a certain fuel-to-oxygen ratio in the cylinder to ensure all of the fuel is ignited. Cold air is denser, so more fuel is injected in colder weather for the same volume of air. While accelerating, it won't make much of a difference, but it would lead to decreased fuel mileage due to more fuel used while idling and coasting.