The company, which conducts its own standardised on-road tests for vehicle fuel economy and emissions, said the average results from the 459 cars it had tested since October 2011 painted a different picture to the improvements claimed by manufacturers.

Average economy according to official figures is 57mpg, it said, increasing 1.7mpg each year, while real-world figures had stayed constant at around 44mpg. The gap between the two, at 22%, is five percentage points greater than it was when the company began testing.  

But this gap varies enormously depending on engine size, from over a third in engines under 1.0 litres in capacity, to almost nothing for the largest engine category – of over 5.0 litres.

Average real-world economy for engines under 1.0 litres was found to be lower than that of the 1,000-1,999cc class, and also the 2,000-2,999cc engines. Diesels were also reported to be slightly closer to manufacturer figures than petrols, a difference of 17% compared to 19% respectively.

Likewise, petrol hybrids, with an average of 47.6mpg for the tests done by Emissions Analytics, recorded the greatest difference compared to official economy, coming in at 22% under the manufacturer figures.