PARIS – French automaker PSA admits some of its official fuel-economy figures are more than 30% overly optimistic.

In a move that should shame other manufacturers, PSA has “come clean” over real-world fuel figures it recorded during tests conducted with two non-governmental organizations, Transport & Environment and France Nature Environment (FNE), and audited by Bureau Veritas.

The test schedule involved 30 of the manufacturer’s core products under its three brands, Peugeot, Citroen and DS.

The worst-performing vehicle was the diesel Citroen C3 BlueHDi 75 S&S 1.6L on 15-in. low-rolling-resistance tires, which recorded a true fuel economy of 47.9 mpg (4.9 L/100 km) against the official claim of 78.3 mpg (3.0 L/100 km).

The best performer was the Peugeot 2008 1.2L PureTech 82 on 16-in. low-rolling-resistance tires that returned a real-world 36.7 mpg (6.4 L/100 km) against the manufacturer’s claimed 47.9 mpg (4.9 L/100 km).

The tests were on a mix of urban (15.5 miles [25 km]), rural (24.2 miles [39 km]) and interstate (19.2 miles [31 km]) public roads under real-life driving conditions. The vehicles carried passengers and luggage loads, and air-conditioning systems were used during testing.

PSA says its findings reflect those reported by its own customers. It is the first major automaker to publish true fuel-economy figures after beginning tests in November 2015 in response to “collapsing consumer confidence in car testing.”

Based on the European Union’s Real Driving Emissions project, the procedure measured fuel consumption using a portable emissions measurement system.

PSA has promised that by the end of 2016 its brands will offer a simulator on their websites allowing customers to predict their vehicles’ fuel consumption based on driving style and conditions. At the same time, an eco‑driving application also will be made available online to help consumers manage their fuel consumption.

PSA says it will publish figures for an additional 20 models by the end of the year and in 2017 will extend its testing to include nitrogen-oxide emissions levels.