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It's an age old question: What is more fuel efficient when driving a car and trying to keep cool, having the air conditioning on or the windows down?

For years people have tussled over which uses more fuel, with most of the opinion that having the windows down is better up to a certain speed, when drivers should then switch to air conditioning.

However, studies suggest that having the windows down could be the best solution at all speeds - and the air conditioning never needs to be turned on.

Air conditioning versus rolling the windows down: which makes your car more fuel efficient? Previously it had been thought it was better to switch to air conditioning at higher speeds, but new research suggests it might always be better to roll the windows down and never touch the air conditioning button

Basic physics suggests that air conditioning should perform better at higher speeds.

A car's air conditioner is essentially powered by the engine - most cars have a compressor system driven by a belt.

When driving at slower speeds, the small but continuous effect of having the air conditioning on means the car's engine works harder, and thus more fuel is used.

At speeds of 70 miles (110 km) per hour on a motorway, however, it is often said that the effect of air conditioning on fuel efficiency is negligible when compared to having windows open.

Depending on speed, having air conditioning on at a medium level can reduce the fuel efficiency of a car down by up to 10 per cent, according to Vox.

At slower speeds it makes more sense to have the windows open to cool the car down - air easily circulates from outside to inside the car without causing too much drag.

As speed is increased, however, the air resistance on the car also increases, meaning the engine has to work harder to maintain the car's speed down the road as it becomes less aerodynamic.

So, at higher speeds air conditioning is surely the winner - at least, that's what was previously thought.

But new research by Vox suggests this is not the case.

A 2004 study by the Society of Automobile Engineers found that Sedans used more fuel when air conditioning was on than when windows were down. Note the axis on the left is gallons per mile, so a higher number is worse performing

As a car's speed increases, so too does its air resistance. A car travelling at 70 miles per hour experiences four times as much drag as one travelling at 35 miles per hour. This might suggest it is better to have the windows up, but research shows that air conditioning is worse for fuel efficiency even at these speeds

Taking a look at previous studies, they found that it was actually more efficient to have the windows down for any speed expected on a road.

In 2004, for example, the Society of Automobile Engineers found that both SUVs and Sedans were considerably less efficient at all speeds up to 70 miles (110 kilometres) per hour when using air conditioning over having the windows down.

A test by popular American TV show Mythbusters, meanwhile, found similar results.

This is despite the fact that drag doubles as speed increases - for example, a car travelling at 70 miles per hour will have twice as much force pushing against it than one travelling at 35 miles per hour.

The reason is that air conditioning eats up more fuel that previously thought, while the effects of drag by having a window open are not as bad as most think.

So it seems the solution to being both fuel efficient and keeping cool as the weather heats up might be to leave the air conditioning off, and crank the window open just a bit.

Of course, if you really want to be fuel efficient you might decide to forego both having the air conditioning on and rolling your windows down - you'll just have to bear the heat while driving.

The Society of Automobile Engineers found the effect was even more pronounced in SUVs, where air conditioning became less and less efficient at higher speeds