It is the little argument on every summer journey: do you switch on the air conditioning or wind the windows down to save fuel? Both choices cost you something, but in different ways, and the right answer shifts above all with your speed. The rule of thumb is simple: around town and at low speed, keep the windows down; on the motorway, shut everything and switch the air con on.
What the air con really costs
A car's air conditioning is driven by the engine and draws power: on average it pushes fuel use up by 5 to 10%, a little more in extreme heat and in the first few minutes after you switch it on. It is a near-constant cost that barely changes with your speed: whether you are doing 20 or 80 mph, the compressor works much the same.
In hard figures, on a car using around 7 litres per 100 km the air con typically adds about half a litre per 100 km. Over a long summer drive that comes to only a couple of pounds of extra petrol at the pump: far less than most drivers fear.
Good to know: a cabin that is too hot tires the driver and dulls concentration. Safety always comes before saving, so on scorching days the air con is money well spent.
Windows: all but free in town, a penalty on the motorway
Winding the windows down looks like the no-cost option, but at speed the rush of air creates heavy aerodynamic drag: the incoming air slows the car and the engine has to burn more to make up for it. Most tests put the break-even point at around 45 to 50 mph: below that, open windows win; above it, the air con does.
The exact figure depends on the car, since a sleek saloon and a tall SUV do not behave the same way, but the logic holds. In town, at low speed, wind resistance is slight and the windows alone are enough to keep you cool at next to no cost.
Electric car: the air con shows up straight away on the range
On an electric car the reasoning is identical, but the effect is obvious at once, because the air con eats directly into your range rather than your fuel tank. In one mixed-route test the same car used about 18.5 kWh per 100 km with the windows down and 19.5 kWh with the air con running.
At motorway speed, though, the ranking flips: around 21.0 kWh per 100 km with the windows open against 20.8 with the air con on. On an electric car too it is your speed that decides which system wins, exactly as on a petrol car.
One extra tip for anyone charging at home: cool the cabin while the car is still plugged in, so you set off cool without touching the battery. It is the same logic as our guide to saving electricity at home: spend energy only where it truly earns its keep.
The moves that count before you even choose
Whichever way you go between windows and air con, a few habits save fuel every time.
- Vent the scorching air before you set off: with a baking car, drop the windows for a minute so the hottest air escapes and the air con has far less work to do.
- Use the recirculation button: it cools the air already inside rather than dragging in the hot air from outside, so it uses less; just switch it off the moment the windows start to mist up.
- Do not overdo the temperature: 22 to 24°C is plenty, far better than setting it to the minimum.
- Park in the shade or use a sunshade: a cooler cabin means less work for the air con.
- Switching the air con off for the last few minutes before you arrive: it saves next to nothing and only costs you comfort.
- Leaving recirculation on too long with the windows misted up: clearing them needs fresh air from outside.
- Setting the temperature to the minimum: the compressor runs flat out for no real gain.