Car air conditioning and fuel consumption: what you need to know

Air conditioning is a major help when summer temperatures climb, but it’s worth knowing it can also push up a car’s fuel consumption.

Air conditioning and fuel consumption: why it can rise

For the system to work, the air-conditioning compressor draws power from the car’s engine - unless you’re driving an electric vehicle. That extra demand increases the load on the engine and, as a result, fuel use goes up.

In some cases, fuel consumption can increase by up to 10%, although the exact change depends on several factors. Short journeys are where the gap between driving with the air conditioning on versus off is typically most noticeable.

Short trips: the air conditioning works harder

The reason is simple. When you set off, you often switch the air conditioning to automatic or close to maximum output to cool the cabin as quickly as possible. On a short run, that means the system spends most (or all) of the journey operating with higher energy requirements.

Why not just open the car windows?

When budgets are tight, any saving helps - so opening the windows might seem like the obvious alternative.

However, there’s another issue: the car’s aerodynamics. Aerodynamic efficiency matters for both stability and fuel consumption, and opening the windows gradually reduces that efficiency.

Predictably, aerodynamic forces have a bigger effect at higher speeds, which is exactly when you’ll see the largest differences in consumption between driving with the windows open or closed.

Still unsure? Picture yourself on the motorway at 120 km/h with the windows down. Besides being unpleasant for your ears because of the turbulence, the extra drag means the engine has to work harder to hold the same speed - and more effort translates into higher fuel consumption.

It’s not easy to find definitive figures for how much consumption increases with the windows open, because aerodynamic efficiency varies a great deal from one vehicle to another.

Even so, the limited tests available consistently show that, at higher speeds, driving with the windows open generally affects fuel consumption more than driving with the windows closed and the air conditioning switched on.

Is the best option somewhere in the middle?

With that in mind, and to reduce the impact on fuel use while tackling the “hell” of a car interior on the hottest days, here are a few practical tips.

Before you set off, start by opening the windows. It’s common for a cabin to reach temperatures 10–20 °C above the outdoor air, and ventilating by opening the windows helps bring the inside temperature closer to the outside.

If your journey is short - both in distance and time - and you’ll be travelling at low speeds, it may be best not to switch on the air conditioning at all. If you do turn it on, you might not even feel much benefit before you arrive.

If the trip is longer, drive for a few minutes with the windows open first, then close them and switch on the air conditioning. You may find you don’t need to push it as hard.

As a general rule, if you’re travelling on motorways or dual carriageways, it’s more efficient to drive with the air conditioning on than with the windows open.