Experts have questioned the validity of official fuel economy figures from manufacturers, with smaller cars seeing the biggest discrepancies.
Drivers have long been told that a smaller vehicle will offer a better miles per gallon (mpg) rate than cars with bigger, more powerful engines.
However, the official figures put out by car makers are calculated under laboratory conditions and don't take account of real world driving.
Tests on 500 vehicles - half petrol, half diesel - each driven for three hours on UK roads, found that vehicles typically travelled 18% fewer miles per gallon than stated in manufacturers' specifications.
Emissions Analytics, the company behind the tests, explained that this was down to accelerating and braking on real roads rather than in the lab.
And the differences were especially large for vehicles with smaller engines. Tests showed that vehicles with an engine size up to one litre had an average advertised 60.3mpg, but consumption was measured at 38.6mpg in tests, a fall of 36%.
This compared to an a verage consumption for cars with one to two-litre engines of 46.7mpg, 21% below the advertised 59.1mpg.
This meant they travelled further on the same amount of fuel than the average smaller car.
And m odels with two to three-litre engines had a result of 45mpg on average, 15% below their advertised average of 52.9mpg.
Professor Stephen Glaister, director of the RAC Foundation , said: "These astonishing figures only fuel the debate on the worth of official mpg data. Well over two million new cars will be sold in the UK this year, with small vehicles topping the sales chart.
"But how many drivers will actually get what they think they have paid for? The answer, in terms of fuel efficiency, must be not many."
Copyright Press Association 2014