How Air Conditioning, Speed and Load Affect Fuel Use in Your Car

Every driver has heard advice such as "turn off the air conditioner to save petrol" or "drive faster to save time". Some of it is right, some is half right and some is wrong. This article looks at the three factors that most often change fuel use in everyday driving, which are the air conditioner, speed and load, and explains how each works so that you can choose sensibly.

Air conditioning

The air conditioner's compressor is driven by the engine, so using it takes power, and that power comes from fuel. The effect is largest at low speeds and in stop-and-go traffic, where the engine is working lightly and the compressor is a bigger share of the load. In hot weather the increase in fuel use can be noticeable, particularly in smaller cars with small engines. How much depends on the car, the outside temperature and how hard the system works, so there is no single figure, but expect a visible difference in your own records.

Windows or air conditioner? At low speeds in town, opening the windows creates little drag, and it usually uses less fuel than the air conditioner. At higher speeds, open windows disturb the airflow around the car and increase drag, so it can be more economical to close them and use the air conditioner. The crossover is not exact, but a rule of thumb is to use the windows in town and the air conditioner on the highway.

Ways to use the air conditioner more efficiently

• If the car has been parked in the sun, open the doors or windows for a minute to let the hot air out before you switch on cooling.
• Start with the fan at a higher setting and use the recirculation mode once the cabin has cooled, so that the system does not keep cooling hot outside air.
• Do not set the temperature lower than you need. A moderate setting is more efficient than the coldest.
• Park in shade or use a windscreen sunshade when you can. A cooler cabin needs less work from the system.
• Have the air conditioning serviced as recommended. A system with low gas or a blocked filter works harder.

Speed

The faster a car moves, the more energy it uses to push through the air. The force of air resistance rises with the square of speed, so going twice as fast requires roughly four times the force. This is why fuel use climbs sharply at high speeds. Most cars are most efficient at a steady moderate speed in a high gear, and efficiency falls as speed rises beyond that.

Consider a 200 km trip. Driving a little slower may add a few minutes, but it can reduce fuel use noticeably. Whether the time saved is worth the extra fuel depends on the value of your time, but many drivers are surprised how little time they gain. For the same 200 km, travelling at an average of 100 km/h takes two hours, while 110 km/h takes about one hour and 49 minutes. That saves roughly 11 minutes, but the fuel cost can be several per cent higher. Our fuel calculator can estimate what that difference costs at the current price.

Speed is also about stability

Constant speed is more efficient than constantly changing speed. Using cruise control on a clear, flat highway can help, as can leaving a larger gap in traffic so that you do not need to brake and accelerate repeatedly. Do not go over the legal limit, and keep a safe speed for the road and the weather. Safety always comes first.

Load and weight

Extra weight means more energy needed to accelerate, climb and overcome rolling resistance. The effect is larger for small cars and in stop-and-go driving, where the car repeatedly speeds up. Check what you are carrying. Old tools, bags, sports gear and bottled water can add up to many kilograms. Remove anything that you do not need on a regular basis.

Roof racks and carriers add weight and, more importantly, drag. A box or a pile of luggage on the roof can increase fuel use at highway speeds. Take them off when they are not needed, and when you do carry a load, secure it firmly and keep it as low and as central as possible.

Putting the three together

Imagine a small car that normally gives 12 km per litre, driven 1,000 km a month. In hard conditions, such as hot weather, constant air conditioning, fast highway driving and a loaded roof rack, mileage might fall to 10 km per litre. The same distance would then need 100 litres instead of about 83. At an illustrative Rs 400 per litre that is about Rs 6,700 more a month. Not every driver meets all of these conditions, but it shows how the factors can stack up.

A sensible approach

1. Use windows at low speed and air conditioning at highway speed.
2. Keep to a steady, moderate speed where it is safe to do so.
3. Remove unnecessary weight and take off the roof rack when you can.
4. Check your tyre pressure, since a heavy load needs the correct pressure. See tyre pressure and fuel economy.
5. Track the results in a fuel log.

Frequently asked questions

Does the air conditioner damage the engine? No, when it is working properly. It adds load, but the engine is designed to handle it.

Is it better to switch off the engine and the air conditioner while waiting? If the wait is long, switching off saves fuel, but think about safety and heat, particularly with children or animals in the car. Never leave anyone in a parked car.

For the habits that matter on every trip, read our guide to fuel-efficient driving habits for cars, and for the price side, see our guide to fuel prices in Pakistan.