As temperatures begin to rise and fuel prices remain a concern, many drivers will be looking for simple ways to stay comfortable without using more fuel than necessary.
In warm weather, motorists usually rely on one of two options to cool the car: opening the windows or switching on the air conditioning. While both can help reduce cabin heat, each affects fuel economy in a different way.
Both Air Conditioning and Open Windows Affect Fuel Use
Many drivers underestimate how much these two choices can influence overall fuel consumption.
Air conditioning places extra demand on the engine or power system, which means the vehicle uses more fuel. On the other hand, driving with the windows open increases aerodynamic drag, especially at higher speeds, which can also reduce efficiency.
The 45mph Rule Is a Useful Guide
Motoring experts say the 45mph mark is a practical guide when deciding whether to use air conditioning or keep the windows open.
At lower speeds, such as in town driving, open windows are often the more fuel-efficient option. However, once speeds rise, open windows create more drag and can force the vehicle to work harder, making air conditioning the better choice in many cases.
It is not a fixed legal or technical cut-off, but it is widely used as a sensible guideline.
Fuel Economy Impact of Each Option
Research and expert guidance suggest the impact can be noticeable if drivers make the wrong choice for the conditions.
| Cooling Method | Typical Impact on Fuel Economy |
|---|---|
| Air conditioning | Can increase fuel consumption by up to 10% |
| Open windows at higher speeds | Can increase fuel consumption by up to 20% |
This means that while air conditioning uses more energy, driving with the windows down on faster roads may actually be the more expensive option in fuel terms.
What Drivers Should Do at Different Speeds
The most practical approach depends on how fast the vehicle is travelling.
| Driving Situation | Better Option |
|---|---|
| Low-speed city driving | Open windows are often more efficient |
| Higher-speed driving | Air conditioning is usually more efficient |
| Mixed driving | Use whichever option best suits the speed and comfort level |
This approach can help drivers stay cooler while also making better decisions about fuel use.
Hybrid Vehicles May Be Affected Differently
Some research has shown that hybrid vehicles can experience a greater fuel economy penalty from air conditioning use than conventional petrol or diesel cars.
Figures cited in motoring analysis suggest average fuel economy reductions of the following:
| Vehicle Type | Average Fuel Economy Drop From Air Conditioning |
|---|---|
| Hybrid | 6.1% |
| Petrol | 3.8% |
| Diesel | 4.6% |
This suggests that drivers of hybrid vehicles may want to be especially mindful of how often and how heavily they rely on air conditioning.
What to Do in Heavy Traffic
In slow-moving or bumper-to-bumper traffic, the decision becomes less straightforward.
At these speeds, open windows may be practical, but outside air quality can be poor because of surrounding traffic. In such cases, some experts suggest using the air conditioning system in recirculation mode so the vehicle cools the inside air without constantly pulling in hot, polluted air from outside.
Other Ways to Keep the Car Cool
Drivers can also reduce the need for air conditioning by keeping the vehicle naturally cooler before they set off.
Useful options include:
| Method | Benefit |
|---|---|
| Parking in the shade | Reduces heat buildup inside the vehicle |
| Using a garage or covered parking | Helps keep cabin temperature lower |
| Window tinting | Can reduce heat from direct sunlight |
| Heat deflectors or sunshades | Helps block or reduce solar heat |
Some guidance suggests window tinting can cut the heat caused by direct sunlight by as much as 70%, which may reduce the need to use air conditioning as often.
Final Overview
Drivers trying to save fuel in hot weather should not assume that one option is always best.
At lower speeds, opening the windows is often the more efficient choice. At higher speeds, air conditioning is usually better because open windows create extra drag. The 45mph rule remains a useful general guide, even though the exact point can vary by vehicle type and driving conditions.
For the best balance of comfort and economy, drivers should adjust their approach based on speed, traffic conditions, and how hot the vehicle is before the journey begins.
