What is the impact of air conditioning use on fuel economy?

Fuel Economy may be affected by many factors, including, load on the engine and vehicle aerodynamics. At highway speeds, your vehicle is generally more aerodynamic and fuel efficient with the windows up and the Air Conditioning (A/C) on than with the windows rolled down and A/C off. At lower speeds (city driving), the impact of drag due to having the windows down is less significant. In such cases, it may be more fuel-efficient to drive your vehicle with the windows down and the A/C system off.

Note: Although it is useful to open windows of your vehicle to expel hot air for the first few minutes of driving, operating the A/C with the windows down beyond these initial few minutes of driving is not recommended.  Leaving the windows open for extended periods, with the A/C on, will diminish effective cooling of the vehicle cabin and negatively affect fuel economy, by increasing the load on the A/C system components.

On the other hand, running the heater has no appreciable effect on fuel efficiency.