CARMAKERS are again in regulators’ headlights over emissions, on both sides of the Atlantic. On January 13th French prosecutors announced they were investigating Renault for “suspected cheating” on diesel emissions. A day earlier, America’s Environmental Protection Agency (EPA) accused Fiat Chrysler (whose chairman, John Elkann, sits on the board of The Economist’s parent company) of using undeclared software in 104,000 diesel-engined Jeeps and Dodge Ram pick-ups. The EPA says that the software increases emissions of nitrogen oxides (NOx) from the vehicles in normal use, and was not declared or justified by the carmaker. It is illegal to fail to inform the EPA of software that might affect emissions, although the agency stopped short of saying it was a "defeat device". In 2015 Volkswagen (VW) admitted installing “defeat devices” in several models that were designed to hide the true level of emissions from vehicles during the testing process. VW has coughed up more than $20bn so far in fines and compensation for the 600,000 American vehicles in question.

America has much tougher laws on toxic NOx emission than Europe, with a limit of 40 mg per kilometre—half of the maximum allowed from European diesel models sold since September 2014. The United States also has far fewer diesel vehicles: around 2% of its cars run on diesel, compared with over half on European roads. Europe’s carmakers still appear to be falling short. The International Council on Clean Transportation, an NGO, compiled results from over 1,200 real-world tests on vehicles that conformed to legal limits in laboratory testing. Its researchers found that the cars from all manufacturers evaluated exceeded the legal limit on NOx many times over. The worst offender was Renault-Nissan. Its vehicles pumped out ten times more NOx on average than the current ceiling. Fiat Chrysler, which includes Jeep, came next. VW’s models, conceived pre-Dieselgate, are among the best performers and have improved markedly since the latest rules came into effect.

The gap between laboratory and real-world results is not limited to NOx. Most cars′ measured fuel efficiency (and hence carbon dioxide emissions) on the open road falls short of their manufacturers′ claims—and the gulf is widening. The EU is introducing a new testing regime this year, which it hopes will force carmakers to control emissions more tightly. Not a moment too soon for the environment.