What Car? tested 56 new petrol, diesel and hybrid models that have been type approved under the new, tougher, Worldwide Harmonised Light Vehicle Test Procedure (WLTP) emissions and fuel economy tests.
It found that on average, new petrol, diesel and hybrid vehicles are 5.4% less efficient than the WLTP figures that manufacturers have to quote, but the research revealed significant differences between models. The worst performers were found to be more than 20% less efficient than their official figures, while the best performers were found to be 15% more efficient.
The tougher and longer WLTP emissions and type approval test was introduced in 2017 by the European Commission and became mandatory for all new vehicles from September 2018 onwards, with the aim of providing more accurate fuel consumption figures for new car buyers.
Steve Huntingford, editor of What Car?, said: "For years, most buyers have taken the official fuel economy figures with a grain of salt. Our research shows that while the latest test is more accurate, there are still large differences on some makes and models, which is why it's important for buyers to do their research before buying."