How are fuel economy figures calculated?

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Will Nightingale
15 February 2008

How are fuel economy figures calculated?

Q: I'm trying to understand exactly how manufacturers give an accurate fuel consumption figure based on various types of driving. Surely it depends entirely on the weather conditions and driving style?
Mark Salt

A: You're absolutely right - fuel consumption in the 'real world' depends entirely on conditions and individual driving style.

Because of this, cars often return markedly different fuel economy figures from the manufacturers' official figures.

The official figures are calculated in a laboratory before the car goes on sale. It's impossible for testers to realistically replicate road conditions, which is why motorists rarely match the MPG figure quoted in the sales brochure.

Although official figures can't be relied upon for accuracy in the real world, they are useful for comparing one car with another. All cars submitted for type approval are tested in the same way, so if one car shows better fuel economy when tested, chances are it will be more efficient on the road, too. This isn't always the case, though.

Also bear in mind that cars are tested when new. Engines lose efficiency over time as components wear, so this means older cars are even less likely to match the official fuel economy figures.