With heatwaves in the headlines, we wanted to know – what's the best way to cool your car?
So we're going to find out… with science!
How we tested air conditioning vs open windows
- We drove a Jaecoo 7 1.6T AWD on the same 46-mile test route in Portsmouth
- We performed the test three times:
- Once with all windows open and the A/C off
- Once with the A/C on and all windows closed
- One control test with the A/C off and windows closed
- We used the same fuel pump at the end of each run to measure how much petrol we used
- We used a data-logging thermometer to record the in-car temperature
- We tested during the middle of the day to avoid rush-hour traffic
Air conditioning vs open windows – summary
- The control test was the most efficient, averaging 41.5mpg
- Air conditioning was 3.9% less efficient, averaging 39.9mpg
- Driving with windows open was 6.6% less efficient, averaging 38.8mpg
- The in-car temperature rose during our control test, peaking at 35.8ºC
- Air conditioning was very effective, cooling the car from 36ºC to 25.5ºC during the test
- Open windows only dropped from 34.7ºC to 33ºC, but was still preferable to the control test
A/C vs open windows – which is more efficient?
On our test route, using the air conditioning was more efficient than driving with the windows open. Our Jaecoo 7 achieved 39.9mpg with the A/C on versus 38.8mpg with the windows open, and 41.5mpg in our control test.
That means fuel consumption was 3.9% higher than our control test with the A/C on, while it was 6.6% higher with the windows open.
A/C vs open windows – which gets more range?
Let's take those economy figures and see what impact they'll have on the car's total range…
Our Jaecoo 7 has a 60-litre fuel tank, so it should be able to cover 547.7 miles on a full tank. That's driving with the A/C off and the windows shut.
If you drove the whole tank with your A/C on, you'd cover 526.6 miles. That's a 21.1-mile penalty compared with our control test.
If you drove the whole tank with your windows open, you'd cover 511.7 miles. That's 36 miles less than our control test.
A/C vs open windows – what are the annual fuel costs?
Let's imagine you really like driving with your A/C on or with your windows open, so you decide to do it all-year round. What impact would that have on your annual fuel costs? We'll assume you cover a fairly average 7,000 miles per year, and refuel at a rate of £1.64/litre.
If you drove all year with your A/C off and your windows closed, you'd spend £1,257.55 on fuel.
Driving all year with your A/C on would cost £1,308.06 – an extra £50.51 over those 7,000 miles.
Driving all year with your windows open would cost £1,345.13 – adding £87.58 to your annual fuel bill.
A/C vs open windows – which cools down the car fastest?
Our testing showed a dramatic difference in how effective these methods were at cooling the cabin.
Air conditioning dropped the interior temperature from 36ºC to 25.5ºC. Plus, most of that cooling was achieved within the first few minutes, leaving our testers a comfortable temperature for the majority of the test.
Driving with the windows open only resulted in an overall drop from 34.7ºC to 33ºC. In addition, the cabin was very blustery and noisy at higher speeds. Despite this, all testers still preferred open windows to the control test.
Speaking of… the control test began earlier in the day with the cabin at 28.3ºC, but temperatures rose steadily throughout and ended at a sweltering 35.8ºC. Testers reported "miserable" conditions inside the car by the end.
You can also see a small temperature bump towards the end of our A/C and open window tests. This is because our test route looped back on itself, causing the sunlight to hit the front of the car and push temperatures up.
A/C vs open windows – which is best?
The answer here varies from person to person. Yes, we can clearly see air conditioning is the more efficient choice, and is definitely more effective at cooling the cabin. However, some drivers just like driving with their windows open, whereas others will prefer to keep the A/C blowing.
Neither option is 'wrong' as it just comes down to your personal preferences. And, while our testing certainly shows a difference between the options, it also shows that the difference is fairly small in the grand scheme of things. So, if you prefer the wind in your hair over the icy chill of air conditioning, you can carry on guilt-free.
What might be worth considering is when and where to either open your windows or fire up the A/C. At slow, urban speeds, the efficiency penalty experienced by open windows is fairly minimal, and the moving air feels like a gentle breeze. At motorway speeds, open windows feel more like a gusty tornado, so you might find using the A/C is more civilised and efficient on faster roads.
Is A/C more efficient than open windows at high speeds?
With more time and resources, we would have liked to test how efficient air conditioning and open windows are at different speeds. This is because open windows are inefficient due to the extra aerodynamic drag they create, so it's reasonable to assume that the efficiency penalty is higher at faster speeds where more drag is created.
Our testers noticed the Jaecoo 7 was harder to keep at 70mph with the windows open. This firmly indicates the car was experiencing greater drag at motorway speeds rather than slower, urban speeds.
While we couldn't replicate it with our test data, we strongly suspect the efficiency gap between A/C and open windows would narrow at urban speeds. There is probably a crossover point somewhere between 30-40mph, where you start to get a noticeable efficiency benefit by choosing A/C over open windows due to the extra drag the latter creates.
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