Drivers of petrol and diesel vehicles are being urged to fill up their tanks before Thursday, July 9, 2026, as the Met Office forecasts a peak in hot weather with temperatures reaching 34C in London. The sweltering conditions are expected to persist across southern England through Saturday, with highs of 32C on Friday and 30C on Saturday. The heat poses an unexpected risk to fuel tanks, causing petrol and diesel to run down more quickly than normal due to changes in driver behaviour and fuel evaporation.
How Hot Weather Affects Fuel Consumption
According to the US Department of Energy, hot weather can initially improve fuel economy because engines warm up faster and summer-grade gasoline has slightly more energy. However, the benefits are quickly offset by increased use of air conditioning (A/C). The department states: "Under very hot conditions, A/C use can reduce a conventional vehicle's fuel economy by more than 25%, particularly on short trips." For hybrids, plug-in hybrids, and electric vehicles, the percentage impact can be even larger. Additionally, drivers tend to open windows, which increases aerodynamic drag and further reduces efficiency.
The RAC reports that current average fuel prices are 150.68p per litre for petrol and 164.76p for diesel. These prices remain significantly higher than before the conflict in Iran, and with news that a ceasefire may be ending, prices are expected to rise again soon. Drivers are advised to fill up before Thursday to avoid wasting fuel and money.
Risks of Running Low on Fuel in Hot Weather
Experts at PassMeFast.co.uk warn that hot weather causes fuel levels to drop faster than usual, making it unreliable to estimate remaining driving time. They advise: "To avoid getting stuck in the middle of nowhere, or at the centre of a traffic jam on a hot day, make sure you head to the petrol station in good time. Playing it close to the line is just not worth it if the sun's out!"
RAC spokesperson Rod Dennis dismissed a myth that filling up in hot weather could cause a vehicle to explode. He said: "There is no truth in this. All fuel systems on passenger vehicles are designed to cope with any expansion of fuel, or vapour coming from the fuel. There is no risk of explosion from filling up a fuel tank fully and drivers should have no concerns in doing so."
Optimal Temperature for Fuel Economy
Fuel Finder UK explains that the 'sweet spot' for fuel economy is between 15C and 20C. At -5C, efficiency drops by around 10% compared to 20C, and for short trips under four miles, the penalty can reach 20% or more. Once temperatures hit 32C, engine efficiency is reduced and there is a risk of overheating. Therefore, while 20C offers better fuel economy than cold weather, the gains are lost as temperatures rise.
Drivers are encouraged to fill up before Thursday to avoid the double impact of higher fuel consumption and potential price increases, ensuring they are not caught out by the heat.
