The Uk-based vehicle emissions data service conducted real-world tests on over 500 model variants, and found that engines under one litre have the greatest variance from their official figures. As well as achieving 36% less than expected in terms of fuel economy, the firm says such cars will only achieve 3mpg more than the average of all the cars it tested, which includes performance cars.

It added that the key to understanding why these “eco” engines perform so poorly on the road lies in the miles per gallon penalty of accelerating.

The firm’s data show shows that the smaller the engine, the larger the penalty acceleration has on mpg. It added: ‘And because the NEDC test has relatively few episodes of acceleration and those that it does have are of a gentle nature, these small engines appear to perform well and receive a high mpg result. That is, until they are taken on the road and driven by real drivers when every rev suddenly burns through the fuel.’

This is in contrast to US data, where the EPA five-test cycle contains much harsher boundaries and although there are large engines and an overall lower average mpg, the variance to the statutory figures is very small.

Emissions Analytics added: ‘You may not get as much from your tank – average European fuel economy is over 40% better than in the US – but at least you know how much not much is.’

It added that the US is at a much earlier stage in the trend towards downsizing than Europe, evidenced by the fact that so far it has tested no vehicles in the category below 1 litre.  Perhaps the more strenuous EPA tests will put a limit on such downsizing – or, at least, downsizing without down-weighting.

In conclusion the firm said: ‘For maximum fuel economy you should look for a 1 to 3-litre engine, as these will return around 45-46mpg. And, to avoid being too disappointed with the result, pick a 2 to 3-litre vehicle as it will be only 15% worse than you were told you could achieve.’