Which gives the best fuel economy: using air-conditioning or opening windows?

Which is more fuel efficient using air conditioning or opening windows?

Fuel economy is a serious business for many drivers. Many will experiment with different driving techniques to see if they can save money by increasing their miles per gallon. And one of those tactics is to turn off the air-conditioning (A/C) and open the windows instead. But does it work? We look at findings from experts to discover the best way to keep cool in hot weather.

Is there a simple answer to this age-old conundrum?

If there was, we’d all know about it. The problem is that below a certain speed, around 50mph, it is estimated that it can be more efficient to drive with open windows. But once a car is travelling faster than this, closing the windows and using air-conditioning is reckoned to be the more efficient approach.

Before we get into detail, does it really matter that much?

Why does air conditioning reduce fuel consumption?

That depends. If a car’s annual fuel bill could be cut by 10 per cent, that could save someone travelling 10,000 miles a year roughly £100 (based on an average of 50mpg and current petrol prices of £1.18). Over such a period of time it may not seem like a big deal. But another way of looking at it is to think of the potential savings during a lifetime of driving. Over 60 years, there’s a potential £6000 saving.

Why does air-conditioning reduce fuel consumption?

At the heart of most air-conditioning systems is a compressor. It’s essentially the pump that circulates the vital refrigerant gas under high pressure to the condenser. When the A/C system is switched on and the compressor is running, it is driven by a belt that is connected to the engine’s crankshaft. This saps a small amount of the engine’s energy, a little like gently applying brakes on a bicycle, reducing its fuel efficiency.

At low speeds, drive with windows open

Although it’s old, a study conducted in 2004 by the Society of Automotive Engineers (SAE), in America, found that up to speeds of 50mph, it was more efficient to drive with windows open than have them closed with the air-conditioning running.

At high speeds, shut the windows and run A/C

Which is more fuel efficient using air conditioning or opening windows?

Once above 60mph, when driving on main roads or those with national speed limits, the SAE study suggested that the comparative loss between windows open and no A/C, and windows closed and using A/C, diminishes. However, much depends on the type of car being driven. The more slippery and aerodynamic its body, the more drag through the air is created by open windows.

For a definitive verdict, ask a Guinness World Record breaker

In 2015 Fergal McGrath clinched a Guinness World Record for averaging 100mpg when driving across 24 EU countries. His car, a standard Honda Civic Tourer 1.6 i-DTEC, travelled 8387 miles using just £459-worth of diesel.

McGrath says other drivers often ask him whether it’s more efficient to open windows or keep them closed and use a car’s air-conditioning. He said: “Based on our experience, switch off the air-conditioning if you’re really serious about saving fuel. We drove nearly 8400 miles around the EU, and from Germany southwards the temperature climbed to about 40 degrees C. We kept the windows open, rather than using the air-conditioning. It had less impact on the fuel economy.” There you have it: proof from a record breaker. That said, it is worth remembering that McGrath is unlikely to have been breaking many speed limits.

A final thought and some advice…

Those prepared to put up with discomfort in the quest for maximum fuel efficiency should drive with the windows closed and the air-conditioning switched off. The ventilation fan can blow cool-ish air into the cabin. But on a sweltering day it may not be enough to prevent drivers or passengers from getting a sweat on. Whatever you choose, remember to run the air-conditioning for a short drive once a week. That will keep it in good working order.

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James Mills

James is a motoring journalist and former magazine editor at BBC Top Gear and Auto Express. He has scooped, reported on and reviewed most new cars of the past 20 years, and currently contributes to the Driving section of The Sunday Times.

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