New car buyers in the UK who trust official, government-sanctioned fuel economy figures will pay an average of £1,000 (circa €1,216) more than they expect on fuel over a three-year period.
That is because the official figures are achieved under laboratory conditions that can rarely be replicated in the real world.
What Car?'s True MPG data is scientifically calculated by experienced engineers, who drive test vehicles over a variety of real roads, including motorways, 'A' and 'B' roads and through towns and villages.
Since launching True MPG two years ago, What Car? has tested almost 400 cars in real-world conditions, using cutting-edge test equipment and achieving economy figures that are on average 19% lower than the government figures.
What Car? Editor Jim Holder said: "What Car?'s True MPG tests are the only reliable source of real-world fuel economy.
"With such a discrepancy between the official figures and our real-world data, it's clear that the official test processes need to be updated so that car buyers can place more trust in the figures they are being told.
"In the meantime, car dealers and the car industry aren't obliged to advise consumers about fuel economy beyond pointing at official statistics.
"However, we'd like to see more transparency at play, with dealers pointing to True MPG to give customers a realistic expectation of what fuel economy they can expect."
In a mystery shop exercise of 100 dealers covering eight major car brands, What Car? found that many dealer staff often either stuck rigidly to official data or avoided answering questions on fuel efficiency altogether.