Does Air Conditioning Really Use More Fuel?

The Cost of Keeping Cool

As the weather warms up, drivers face a classic dilemma: does running the air conditioning burn more fuel than simply rolling down the windows? With fuel prices constantly fluctuating, knowing the right answer can make a noticeable difference to your MPG.

How AC Affects Your Engine

Your car's air conditioning compressor is powered directly by the engine via a serpentine belt. When you turn the AC on, it puts an extra physical load on the engine. To compensate, the engine burns more fuel. Studies show that running the AC on a hot day can reduce your fuel economy by anywhere from 5% to 10%.

The Window-Down Dilemma

If AC uses fuel, then rolling the windows down must be free, right? Not exactly.

Opening your windows completely destroys your car's aerodynamic profile. It creates massive "aerodynamic drag," acting like a parachute pulling your car backward. Your engine has to work much harder to push the car through the air, burning extra fuel in the process.

The Golden Rule of Cooling

Automotive engineers have tested this extensively, and they found a clear breaking point where aerodynamics overpower the AC compressor: 45 mph.

• Under 45 mph (City Driving): Aerodynamic drag is low. Rolling your windows down is the most fuel-efficient way to stay cool.
• Over 45 mph (Motorway Driving): Aerodynamic drag becomes massive. It is significantly more fuel-efficient to roll the windows up and turn the AC on.

By following this simple rule, and checking FindPetrol before you fill up, you can ensure your summer road trips are as cheap as possible.

Prices vary widely between sites even within one chain - see current Esso fuel prices across the UK.

Cheaper fuel beats better mileage

Driving technique is worth a few percent on consumption. Choosing the right forecourt is often worth more. With more than 10p a litre between the cheapest and dearest stations in most towns, a two minute check before you set off usually saves more than a month of careful driving. We publish live prices for 2,283 UK towns - find fuel prices near me or browse prices by region.