Yet driving style is not the only reason why real-world fuel efficiency falls short – it’s what motoring engineers put in and what laboratory tests leave out that’s the problem.

“The urban environment in 2006, whether in London or Paris, is significantly different now than it was 25 years ago,” says Julien Bryan of Millbrook Proving Ground in Bedford, which performs emission and fuel testing on more than 900 cars each year. “That’s not what gets measured by the Government.”

Bryan says the EU fuel consumption tests, set up in 1980 to give new car buyers a comparison on fuel economy and carbon dioxide emissions, were designed for “easy driving” and don’t reflect the higher performance and higher speeds of today’s cars. “Cars today do 220kph (137mph) on the motorways. Even a 1.9 litre Fiat will do 200 clicks (124mph).”

In 1995 dealers began displaying three fuel figures instead of one. The new urban and extra-urban numbers, along with their combined average, recognised faster city driving and the greater mix of speeds drivers encounter on roads with less congestion. Today technicians calculate fuel consumption by measuring the amount of carbon dioxide, nitrogen oxide and other hydrocarbons emitted from the exhaust pipe.

“A number of people would argue it [the test] is contrived, but it does give the ability to compare on a consistent basis,” says Simon Barnes, an engineer at the Society of Motor Manufacturers and Traders. “If you drive the car properly, it will achieve the proper results.”

What, then, is proper driving? Barnes claims it means changing gears at 2,500rpm (2,000rpm for diesels), anticipating stops and accelerating gently. Yet even Toyota Prius owners whirring on electricity in traffic-clogged Oxford Circus probably won’t come anywhere near its “official” combined 65.7mpg on the M4.

“In terms of real-life mileage, it depends whose life it is,” says Toyota spokesman Scott Brownlee. “Because a hybrid is especially good when not going quickly, that’s not necessarily represented in the test. Certainly 70mph on the highway isn’t the best for a hybrid.”

Other experts claim the test isn’t designed to tell the whole story.

“The EU cycle is not intended – and no one is trying to pretend – that it’s the way you drive your car, because everyone drives differently,” said Richard Adams of the Motor Industry Research Association.

Adams says many drivers misunderstand the split between the urban and extra-urban cycles and incorrectly measure their mileage. Bryan blamed car manufacturers for “playing the game” – tuning vehicles just to pass the test. “If your sales are based on fitting the tests and if your customers are getting taxed on that, then you’ll make sure it performs best at that output.”

Adams admitted some manufacturers, in a process known as “cycle beating”, can tailor their engines to the light acceleration and low speeds of the tests. However, he said production cars with remapped ECUs – the electronic brains that control emissions and engine performance – are illegal and “more of an urban myth than fact”.

Yet Bryan, whose company developed cleaner and less thirsty engines for London buses, thinks inefficient catalytic converters are partly to blame for lower mileage.

Small catalytic converters, which tend to get hotter faster than larger ones, can suddenly burn lots of fuel without the driver noticing. “In the blink of an eye, you can go from 35mpg to 25mpg, just like that.”

Across the Atlantic, the Environmental Protection Agency has been watching 500 ordinary drivers in Kansas City, Missouri, for more than a year. It is improving the US fuel test for 2008 so drivers will meet the sticker numbers, and even exceed them. The EPA says the new numbers will be five to 20 per cent lower than current figures.

“Over time, it [the test] started showing signs of age as people drive differently. Fuel economy has been fairly low on the priority of most car buyers for decades,” Millett says. “But Americans are becoming more aware, especially as prices go to the $3 per gallon level.”

Since Europeans favour light, compact cars and don’t drive super-sized SUVs and pickup trucks, they can drive further on a tank than most Americans. But does this mean the EU test should be left alone? If anything, Millett says, it’s the drivers who should avoid holding estimates to the fire.

“Of course you’re going to have a wide variability. It’s a sticker, not a crystal ball.”