The old summer argument, air conditioning or windows down, turns out to have a real answer, and it is not the same answer at every speed. Air conditioning costs fuel; open windows cost aerodynamics; and which one is cheaper depends on how fast you are going. Knowing the crossover, and the free tricks that cut the heat before either comes into play, keeps you cool for the least fuel.
The short answer. Air conditioning typically costs around 4 to 8 per cent in fuel, more in stop-start traffic. Open windows cost nothing directly but add aerodynamic drag that grows sharply with speed. The rule of thumb: below about 40 to 50 mph, windows down is the cheaper way to stay cool; at motorway speed, air conditioning wins because drag dominates. Either way, dump the trapped heat before you set off and park in the shade, so you need less cooling in the first place.
What the air conditioning costs
Air conditioning is not free, because the compressor that cools the air is driven by the engine, so it burns a little extra fuel whenever it runs. The penalty is modest on the open road and larger in slow traffic, where the engine is working less but the compressor still has to run. On a hot day it can shave a noticeable slice off your economy, a few per cent on most cars and a touch more on a hybrid, whose efficient engine feels the extra load proportionally more. The same idea governs cooling anywhere, which is why an air conditioner at home is such a heavy load too: removing heat takes real energy, in a car as in a house.
| Vehicle | Typical fuel economy hit from air con |
|---|---|
| Petrol | ~4% (around 1 to 4 mpg on a hot day) |
| Diesel | ~5% |
| Hybrid | ~6% |
Indicative averages; the real figure depends on the car, the heat and how hard the system works. The penalty is largest in slow, stop-start traffic and smallest at a steady cruise.
What open windows cost
Opening the windows feels like the free option, and at low speed it very nearly is. But moving air past an open window creates drag, and aerodynamic drag rises steeply with speed, so the faster you go the more those open windows cost you in fuel. Around town the effect is tiny. On the motorway it can rival or even exceed the cost of the air conditioning, because the car is fighting noticeably more wind resistance. The evidence varies between vehicles, with a boxy car losing more than a sleek one, and some careful tests finding little difference at all on certain cars, but the direction is consistent: the penalty for open windows grows with speed, while the penalty for air conditioning stays much the same.
The rule of thumb, by speed
Put the two together and a simple guide emerges. At low speeds, around town and up to roughly 40 to 50 mph, open windows are the cheaper way to stay cool, because the drag they add is small and you avoid the air-conditioning fuel penalty entirely. At higher speeds, on faster A-roads and the motorway, air conditioning wins, because aerodynamic drag now dominates and keeping the windows shut to stay slippery is worth more than the fuel the compressor uses. So the fuel-savvy approach is windows down in town, windows up and air conditioning on for the motorway. Larger, boxier vehicles cross over at a lower speed than small aerodynamic ones, but the principle is the same for all of them.
Cut the heat before you cool it
The cheapest cooling is the heat you never let build up. A car left in the sun becomes far hotter inside than the air outside, so the first job is to dump that trapped heat before you drive: open the doors or windows for a minute to let the oven-hot air out, and you start from a far lower temperature that needs much less cooling to manage. Park in the shade whenever you can, use a reflective windscreen sunshade to stop the dashboard and cabin baking, and leave the windows cracked a little when parked so heat does not build to the same extreme. Do these and whichever cooling method you then use, windows or air conditioning, has far less work to do, which is where the real fuel saving lives.
Use the cooling well
However you cool the car, a little restraint helps. Set a comfortable temperature rather than the coldest setting, because as with a home thermostat the harder you ask the system to work the more it costs. Once the cabin is cool, switching to recirculate keeps the already-cooled air going round rather than dragging in fresh hot air to chill from scratch, easing the load. And there is no point running the air conditioning in a stationary car with the engine idling just to cool down before you leave, since you burn fuel going nowhere; venting the hot air and driving off gets you cool sooner for less.
Electric cars: the same logic, in range
If you drive an electric car the trade-offs are identical, except the cost shows up as lost range rather than fuel. Air conditioning draws from the battery, open windows add the same speed-dependent drag, and the same low-speed-windows, high-speed-air-con rule applies. The one extra trick an electric car gives you is pre-cooling the cabin while it is still plugged in, so the initial blast of cooling comes from the mains and costs you no range at all, which the EV range in hot weather guide covers along with the rest of the summer range picture.
The bottom line
Staying cool need not cost much fuel if you match the method to the speed: windows down around town, where they are nearly free, and air conditioning on the motorway, where staying aerodynamic matters more than the few per cent the compressor uses. Better still, attack the heat before either: vent the trapped hot air, park in the shade and shade the windscreen, so there is far less heat to fight. Cool the car the smart way and a summer of driving costs barely more than a mild one.