Drivers are being urged to fill up before Thursday where possible as hot weather conditions return in several UK areas, which could see you use more fuel than normal, catching unsuspecting drivers out. The Met Office has forecast a return to hot weather starting this weekend and peaking on Thursday in London and various other parts of the UK, particularly the south of England.
Temperatures could hit 31C on Sunday, August 9, rising to 36C on Thursday, the forecaster has predicted, in what will be the fifth heatwave of summer 2026. Drivers may not realise that hotter weather poses an unexpected risk to do with your fuel tank, which could catch you out. During hot weather, your petrol - or diesel - actually runs down more quickly than normal thanks to both driver behaviour and the car itself, as when it's very hot it can even affect fuel economy.
Fuel Prices and Consumption
Fuel prices have surged upwards in recent months due to the conflict in Iran, and despite a recent slight cooling at the forecourts, remain significantly higher than before. Currently, petrol is 161.47 per litre on average, and diesel is 181.45p according to the RAC, so drivers will not want to waste a single drop.
Cars use up more fuel during hot conditions, both directly and through driver behaviour. In this way, your fuel gauge may actually become inaccurate and not give a proper indication of how much petrol or diesel you have left. According to the US government, drivers tend to take up more behaviours that run your petrol down when the weather is warmer, such as opening windows and using air conditioning.
Impact of Air Conditioning
The US Department of Energy says: "Hot weather can increase fuel economy - the engine warms up to an efficient temperature faster; summer grades of gasoline can have slightly more energy; and warm air causes less aerodynamic drag than cold air.
However, keeping comfortable in hot weather by rolling down the windows or using the air conditioning (A/C) can reduce fuel economy. Running the vehicle's air conditioning is the main contributor to reduced fuel economy in hot weather. Its effect depends on a number of factors, such as the outside temperature, humidity, and intensity of the sun.
Under very hot conditions, A/C use can reduce a conventional vehicle's fuel economy by more than 25%, particularly on short trips. The A/C's effect on hybrids, plug-in hybrids, and electric vehicles (EVs) can be even larger on a percentage basis."
Avoid Getting Stranded
The other reason to fill up before warmer weather is because you don't want to get stranded in traffic on a hot day. Experts at PassMeFast.co.uk explain: "When it comes to topping up the petrol in your car, there are two kinds of people: those who refill as soon as the fuel tank gets to around a quarter full, and those who are happy to drive around with the fuel warning light on.
The thing is, when the weather's nice and warm, your car's fuel levels drop a lot quicker than usual. This means you can't rely on your normal estimations of how much time you have left before needing to fill up.
To avoid getting stuck in the middle of nowhere, or at the centre of a traffic jam on a hot day (both nightmare scenarios), 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!"
Explosion Myth Debunked
Drivers have also in the past been told not to fill up their car during hot conditions due to a risk of the vehicle 'exploding' in the sun. Thankfully, this is not a real problem and is, in fact, a myth.
RAC spokesperson Rod Dennis 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
However, in terms of running a car as economically as possible, there is a sweet spot in between cold weather and hot weather where performance is optimal, say experts. 20C will actually give you better fuel economy than very cold temperatures, but as the temperature rises, the gains start to be lost again.
Fuel Finder UK explains: "According to official fuel economy tests, your car's efficiency drops by around 10% at -5°C compared to 20°C. For short trips under four miles, that penalty can reach 20% or more." According to Fuel Finder, the 'sweet spot' for fuel economy is between 15C and 20C, but once you hit 32C, engine efficiency is reduced and there comes a risk of overheating.
Heatwave Forecast
Met Office Deputy Chief Forecaster Mike Silverstone said: "Temperatures will climb through the weekend and, more significantly, through much of next week. After a brief dip on Monday, high pressure will move across the UK before settling to the east, allowing a southerly flow to develop.
A fairly widespread heatwave is likely across large parts of England and Wales, with hot conditions developing in places. The heat is expected to peak for much of England, Wales, Northern Ireland and Scotland on Wednesday, with up to 34°C possible in parts of central and southern England and southeast Wales. East Anglia and south-east England, however, could see temperatures even higher on Thursday."
