The gap between diesel cars’ official and real-world fuel economy grew to 28% last year, new figures from TMC indicate.

TMC analysis indicates diesels will use an extra £1,800 fuel over 60,000 miles compared to advertised figures.

The analysis of 21,200 fleet cars compared each individual car’s official mpg figure with its actual performance, based on TMC’s database of fuel transactions and mileage reports.

Although overall advertised consumption of the diesels in the sample improved from 62mpg in 2015 to 66mpg in 2016, their economy under real-world conditions improved by only 0.9mpg – bringing the economy gap up from 25% in 2015 to 28% last year. TMC added that the ‘real-world’ gap would equate to an extra £1,800 in fuel and 2.4 tons more CO2 per car over a typical fleet leasing cycle of 60,000 miles.

The same analysis showed a 19% shortfall for petrol cars, with drivers in the sample achieving 9mpg less than their official fuel economy figure.

Paul Hollick, managing director of TMC, said: “The question businesses should ask themselves is whether it is still commercially or environmentally sustainable for them to rely on official figures that are 30% adrift when, these days, they can easily build fully-accurate, real world data about their fleet assets into policy and operational decisions. Of all the factors affecting real-world fuel economy, drivers are undoubtedly the largest influence. Our data enables employers to identify the worst performers and target them for training or other methods of improvement.”

Although Hollick welcomed the forthcoming move to a new mpg/CO2 test cycle, he cautioned that official mpg and CO2 benchmarks will continue to be obtained under artificial conditions.

“Nothing can beat the value of real-world information from your own assets,” he added. “In 2017, every fleet can obtain the tools to capture it and use it to drive cost savings and higher productivity.”