When asked this question, many would answer that smaller cars are more efficient, but road tests confirm that the smallest cars aren’t necessarily the most fuel efficient.
Drivers looking for savings at the petrol pump could be making a mistake if they swap their estate or 4×4 for a smaller car, according to research which suggests that fuel economy estimates are biased against larger vehicles.
Motorists are usually advised that smaller cars can travel more miles per gallon (mpg) than those with larger engines, making them cheaper and more environmentally friendly to run.
But manufacturers’ estimates of fuel economy, based on official laboratory tests, may not reflect the reality when the vehicles are driven on the road.
Tests on 500 vehicles, half petrol, half diesel, each driven for three hours on roads in Britain, found that the cars travelled on average 18 per cent fewer miles per gallon than stated in manufacturers’ specifications.
Emissions Analytics, a data company which measured the cars’ fuel consumption and emissions, explained that this was due to cars accelerating more and travelling at higher speeds on the road than in official testing regimes.
The discrepancy between manufacturers’ claims and the road data was especially stark for vehicles with smaller engines, which generally have to work harder to accelerate.
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 drop of 36 per cent.