The publication has used experienced engineers to scientifically calculate real-life mpg figures by driving drive test vehicles over a variety of real roads, including motorway, A and B-roads and through towns and villages.

In total almost 400 cars were tested in real-world figures, achieving economy figures that are an average of 19% lower than the government figures, according to What Car?

What Car? editor Jim Holder said: ‘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.’