THAT Americans drive brash gas-guzzling cars whereas Europeans putter around in fuel-sipping runabouts is as enduring a stereotype as any. Strangely, this divide holds true even for similar models sold on either side of the Atlantic. European cars will apparently go much farther on a tank of fuel and, as a result, pump out far less carbon dioxide. But the claims in carmakers’ glossy marketing literature bear little relation to actual performance. The basis for fuel-efficiency claims and the regulation of CO2 emissions are results obtained under “type-approval” tests. What was once a gap between the mileages achieved on test tracks and real-world roads has become a chasm, according to a recent report from Transport & Environment (T&E), a green pressure group. Analysis by the International Council on Clean Transportation, a consultancy, of data reported by car owners in Europe shows that in 2013 fuel-economy figures “on the road” were on average 38% worse than those advertised (see chart).

Europe’s procedure is out of date and open to abuse. Carmakers send prototypes for testing. They are engineered to be as frugal as possible. Weighty extras such as the sound system and even wing mirrors are routinely jettisoned. Special lubricants make the engines run more smoothly. Tape on the cracks around panels and doors reduces drag. Low-resistance tyres filled with special gas add to the miles covered.

The cars are driven to a preset routine of gentle accelerations and low speeds, run at the highest permitted temperature of 29ºC (engines are more efficient in the heat). Modern electronics can even detect the pattern of the start of the test and switch into a special “economy mode” that makes for even lower emissions.

The tests are overseen by national regulators, but carmakers pay specialist firms to carry them out. These companies compete for business by promising to “optimise” conditions. One popular test track in Spain is at high altitude (the thinner air reduces aerodynamic drag) and has a surface so smooth that it alone improves efficiency by three percentage points.

Carmakers defend the system, pointing out that a realistic test is tricky as motorists have differing driving styles. But Europeans get neither an accurate picture nor a basis for comparison. Daimler, BMW and Ford are much better than rivals at beating the real-world figures.

Car buyers in America have it better. The test cycle is more consistent with genuine driving habits. Carmakers test their own vehicles. Their results are later checked by the Environmental Protection Agency. Transgressors face stiff punishment; Hyundai-Kia recently settled to pay $350m for overstating fuel efficiency on several models by a few miles per gallon.

Almost all carmakers have met European targets for reducing carbon emissions several years ahead of the deadlines. But this is, in part, illusory, according to T&E. It estimates that between 2008 and 2013 average emissions in Europe fell by 31 grams per kilometre (g/km), but that over half the improvements came from sleight of hand rather than technological progress.

The European Commission wants to introduce a new test by 2017 that more accurately mirrors real driving. Carmakers in Germany and France are pressing their governments to delay the measure. A more stringent test will mean that Europe’s target for average carbon-dioxide emissions of 95g/km by 2021 will have to be revised upwards (the French and German governments are already seeking to delay the next target, which is to take effect in 2025). At a stroke, Europe’s green credentials will look as battered as an old banger.