Motorists could save over 4p per mile by pressing one simple button found in every vehicle during the latest summer heatwave. Opening car windows may seem obvious with the hot weather, but keeping the glass closed could keep costs down. Open windows can increase aerodynamic drag at higher speeds, impacting on the workload of the car and therefore increasing petrol and diesel fuel usage.
Fuel savings at higher speeds
Mike Fazal, motoring expert and CEO of Leasing.com has stressed that keeping windows open at higher speeds could increase fuel consumption by up to 20%. Data from the RAC shows that average unleaded petrol fees stand at 159.97p per litre. Meanwhile, an average petrol car has an average miles per gallon ratio of 36mpg.
Under normal conditions, motorists would therefore be averaging 20p per mile. But with the windows open, MPG rates could rise by 20% to 43.2 miles per gallon. In this situation, road users would actually pay 24p per mile to use the roads in a blow to those trying to watch their wallets.
The 45mph rule
Mike Fazal, motoring expert and CEO of Leasing.com explained: “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. At that point, using the air conditioning can actually be the more fuel-efficient option.”
However, this advice confusingly flips at lower speeds, with road users told to do the exact opposite if they are travelling under 45mph. Fazal explains, "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.
He added: “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%.”
Ventilation and safety
Despite the advice, motorists must still make sure their cabin is in a comfortable condition before setting off. Rule 237 of the Highway Code states that road users must keep their vehicle well-ventilated to avoid drowsiness. It could be considered careless or dangerous driving if the heat is compromising motorists' ability to safely operate their vehicle.
