Drivers can save up to 2p per mile by pressing the air conditioning button on their dashboard when travelling at higher speeds, according to motoring expert Mike Fazal, CEO of Leasing.com. The advice comes as the UK experiences a long hot summer, with fuel costs ranging from 13p to 19p per mile for petrol and diesel vehicles, and 2p to 7p per mile for electric cars.
The 45mph rule for fuel-efficient driving
Fazal explains that open windows create aerodynamic drag at speeds above 45mph, particularly on dual carriageways and motorways, forcing the engine to work harder. “Once you’re travelling faster than around 45mph, especially on dual carriageways or motorways, open windows create aerodynamic drag that makes the engine work harder,” he said. “At that point, using the air conditioning can actually be the more fuel-efficient option.”
He added: “A simple rule drivers can follow is the 45mph rule. Below that speed, it’s usually more fuel efficient to open your windows and let air flow through the cabin.” This guidance suggests that drivers should switch between windows and air conditioning based on their speed to maximise fuel economy.
Impact on fuel economy and costs
The potential savings are significant. With windows open at higher speeds, fuel consumption can rise by up to 20%, pushing petrol and diesel costs to around 21p per mile. In contrast, using air conditioning increases fuel use by only about 10%. “It might seem like a small choice, but it can have a noticeable effect on fuel economy. Air conditioning can increase fuel use by around 10%, while open windows at higher speeds can increase it by up to 20%,” Fazal noted.
This means that at higher speeds, choosing air conditioning over open windows can save approximately 2p per mile, based on the average fuel costs cited. For a typical driver covering 10,000 miles a year, that could translate into annual savings of £200, though the exact amount depends on driving conditions and vehicle type.
Black cars absorb more heat
Drivers should also be aware that vehicle colour affects interior temperatures. Black cars absorb up to 90% of sunlight, which can raise cabin temperatures by 5C to 10C compared to lighter-coloured vehicles. According to experts at RAC, “black cars are hotter in the summer compared to other colours because they generally gain more sunlight than lighter-coloured vehicles as darker surfaces absorb more heat energy.” As a result, black cars can become “hotter both externally and internally when parked in direct sunlight.”
This means that drivers of black cars may need to use air conditioning more aggressively to cool down, which could offset some of the fuel savings. However, the 45mph rule still applies, and using air conditioning at higher speeds remains the more efficient option compared to open windows.
