The UK is set for its third heatwave of the year this week, and drivers are being urged to follow the 45mph rule09:58, 07 Jul 2026Motorists are being advised to stick to a particular rule this week while driving as the UK braces itself for another bout of 30C heat. The Met Office has forecast that the third heatwave of the year will deliver an extended period of hot and dry conditions.The weather is expected to be less muggy than June's heatwave, with lower maximum temperatures and fewer exceptionally balmy nights. Temperatures are set to rise into the low 30s Celsius throughout much of southern England from early in the week, with some areas potentially hitting 34-35C later on.Met Office Deputy Chief Forecaster Steven Keates commented: "Parts of the UK are entering heatwave conditions: the third heatwave in the UK so far this year. However, unlike the May and June heatwaves, we are not expecting this heatwave to be record-breaking."The sweltering conditions mean people will need to take precautionary steps to prevent heat-related illnesses or dangers, such as wearing suncream, keeping well-hydrated and remaining in shaded spots. For motorists, there are extra guidelines to observe, not just to keep cool for their own wellbeing, but also to conserve fuel.Mike Fazal, motoring expert and CEO of Leasing.com, suggests drivers can save fuel with the '45mph air-con rule' this summer. With the third heatwave of the year approaching, many motorists will be seeking ways to stay comfortable while driving, reports the Express.While car air conditioning seems like the obvious answer, understanding when to activate it could genuinely help conserve fuel. 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. Once 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 become the more fuel-efficient choice." Research indicates air conditioning can boost fuel consumption by roughly 10%, while driving with windows down at higher speeds can raise fuel usage by as much as 20%.Content cannot be displayed without consentAnother crucial factor drivers must consider when taking to the road in summer is their choice of footwear. Warm weather frequently sees many motorists switching their regular shoes for flip-flops, clogs or Crocs, but not all summer footwear is appropriate for driving.Anything that slides off readily or makes it trickier to sense the pedals could compromise your command of the vehicle. Fazal says: "Drivers need to be in full control of their vehicle at all times."If your footwear slips, becomes caught or makes it harder to operate the pedals safely, it could contribute to a careless driving offence. While flip-flops and Crocs might be comfortable in hot weather, they're not the best choice behind the wheel. Over summer try keeping a pair of trainers in the car and changing into them before setting off."Those found guilty of careless driving offences face between three and six penalty points alongside fines reaching £5,000, while more serious incidents could lead to five to nine penalty points, a driving ban and, in the worst cases, unlimited fines.