As the UK braces for another heatwave with temperatures forecast to reach 35°C on Wednesday, July 8, motorists are being advised to follow the '4-20 rule' to cool their cars more efficiently and reduce strain on their air conditioning systems.
What Is the 4-20 Rule?
The trick involves opening all four windows and driving for approximately 20 seconds before winding them back up and switching on the air conditioning. This allows the superheated air trapped inside the vehicle to escape naturally, reducing the workload on the AC system. By removing the hottest air first, the system can reach a comfortable temperature more quickly.
According to the US Department of Energy, letting hot air escape before using the air conditioning reduces demand on the system. They also advise drivers not to idle with the air conditioning running before setting off, as this wastes fuel.
Fuel Economy Impact
Running the air conditioning is one of the biggest factors affecting fuel economy during hot weather. The Department of Energy notes that using air conditioning in very hot conditions can reduce a conventional petrol or diesel vehicle's fuel economy by more than 25%, particularly on shorter journeys. The 4-20 rule helps mitigate this by reducing the initial heat load.
Once the hot air has been vented, drivers should roll the windows back up and allow the air conditioning to cool the now less-heated cabin. At higher speeds, this is also the more fuel-efficient approach, as driving with the windows open increases aerodynamic drag.
Additional Tips for Keeping Cool
Experts also recommend using the recirculation setting once the cabin begins to cool. This allows the system to recycle cooler air already inside the vehicle rather than constantly pulling in warmer air from outside, helping maintain a lower cabin temperature.
If possible, drivers should park in the shade or use a reflective windscreen sunshade to reduce heat build-up while the vehicle is stationary. Even cracking the windows slightly when it is safe to do so can help lower cabin temperatures before returning to the car.
Expert Endorsement
While the 4-20 rule is widely shared by motoring experts, the exact claim that it cools a car twice as fast depends on the vehicle, weather conditions and air conditioning system. However, the wider advice to vent hot air before switching on the air conditioning is supported by automotive experts and official energy guidance as an effective way to cool a hot car more efficiently.
