If you live in the UK right now, you are living in the midst of the third heatwave of 2026 in the country. With houses built to retain heat, and with no air-con and high humidity, summers in the UK can be uncomfortable. UK drivers are finding that out too, but for slightly different reasons.
Drivers in the country are being told to stick to a certain rule, as the heat continues to cover much of the country. Everyone in the UK is taking precautionary steps, whether that be more sunshield, staying hydrated, or staying in shady spots when outside.
Drivers, however, are being told to stick to a rule called the ’45mph air-con’ rule. This is related to fuel saving, and as we know, using the air-con in a car can dramatically impact the fuel economy. But it is something no one will want turned off in a heatwave.
Mike Fazal, motoring expert and CEO of Leasing.com, recently explained to British newspapers what this means and why it is a good thing for drivers to stick to. With fuel prices still higher than they have been in the UK, any fuel saved will make the next fill-up a bit cheaper.
What Is the 45MPH Air-Con Rule?
A heatwave will build across England and Wales this week, with temperatures in the low to mid 30s Celsius each day 🌡️
Also warming up across parts of Scotland and Northern Ireland later this week ☀️ pic.twitter.com/fSis0XDwDk
— Met Office (@metoffice) July 6, 2026
Fazal explained that this rule is about when a driver should have the air-con turned on in the car. Doing so at the right time could have a big impact on the fuel you save. “A simple rule drivers can follow is the 45mph rule,” explained Fazal.
“Below that speed, it’s usually more fuel efficient to open your windows and let air flow through the cabin. Once travelling faster than around 45mph, especially on dual carriageways or motorways, open windows create aerodynamic drag that makes the engine work harder,” he added. As we know, more drag means the car is using more fuel to try and maintain the speed you want it to travel at.
But Fazal said that, as you get up to speed on motorways and faster roads, shutting the windows and putting the air-con on is more fuel efficient. In fact, it can boost fuel economy by as much as 10 percent versus having the windows open at that speed. Having the windows down could raise fuel consumption by around 20 percent in some cars. If your car has air-con, this is the way to stay cool in such hot weather.
UK Drivers Have To Wear the Right Shoes
Footwear choices are other factors for UK drivers to consider when the weather is hot and they are off for a summers drive. The problem is, most drivers want to wear things such as sandals or Crocs, shoes that leave your feet in the open air and easily able to cool off, especially with air-con blowing all over them. However, there are drawbacks to using these types of footwear.
As Fazal explained, your feet might slip on the pedals, so it is harder to control the vehicle safely. That could lead to a careless driving offense, worth up to £5,000 or $6,690. One suggestion is to keep trainers in the car, allowing you to drive in normal shoes, but you can then change into something more comfortable when you reach the next destination.