How much do air-con, roof boxes and trailers increase your fuel bill?

05 August


We take a look at the real impact...

For many professional drivers fuel efficiency is a crucial part of their business. Increasing or decreasing your vehicle’s mpg can have a profound effect on your profit.

That’s why Allstar has researched the fuel cost of major motoring accessories - to give drivers and fleet operators a guide to their true fuel cost.

Every piece of powered car equipment, like a heater or lights, uses a little bit of power, but usually it’s negligible given that the way you drive can influence your fuel consumption by a whopping 35%.

So we’ve concentrated on the big enemies of your fuel bill: trailers, roof boxes and air conditioning. Our aim was to answer the question: which is the most costly in terms of your fuel consumption?

Before we give you the results, please note that many of the results and tests we’ve researched have been conducted by an unscientific mix of motoring journalists and car enthusiasts. There are no official figures. The results can be affected greatly by weather and type of vehicle.

So, here are the findings. In reverse order, these are biggest enemies of your fuel bill:

  • Trailer: Surprisingly, results show that a small trailer may only produce a small increase in fuel consumption. Towing one can actually reduce consumption. The law requires drivers with a trailer to keep beneath 60mph. This alone reduces many users’ consumption.

Nevertheless, any increase in weight uses more fuel at any given speed. The rate is roughly a 2% increase for every extra 45kg, so even an unladen trailer could add 4% to consumption.

On a typical 100-mile journey this could cost an extra 64p.

  • Air conditioning: recent studies have show that air con is more of a drain on engine power than previously thought. Studies have shown that at any speed, despite the increased air resistance, it’s better to have your window open than the air conditioning on. Pressing that ‘on’ button can cost you up to 10% more fuel.

On a typical 100-mile journey this could cost an extra £1.60p.

  • Roof box: The worst culprits in fuel cost are roof boxes. One report found an increase in fuel consumption of 50%. More typically drivers are spending between 10 – 25% more on fuel because of their roof box. This is because increasing the ‘frontal area’ of the vehicle destroys the aerodynamics. Even an empty roof rack can add up to 15% to your fuel bill. The added weight and air resistance of a roof box, bikes or luggage strapped to the roof makes it worse. The higher the speed and the smaller, more aerodynamic the vehicle, the greater the effect.

On a typical 100-mile journey a 25% increase in consumption could cost you an extra £4.

  • Save money by driving more efficiently: here’s some advice from the experts at the AA
  • Which is more fuel efficient - opening your car window or using air conditioning? The answer lies in this article:car windows more fuel efficient.
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