Doctor Diesel

Official fuel economy figures

Robot Driver1

The intention of the much-derided EC figures is quite clearly stated as being to give car buyers “comparative information about fuel consumption of different models in standard tests.”

As we well know, it’s rarely possible to match the test figures in real life motoring but many doubt whether they even offer valid comparisons between different cars. The test methods are clearly laid down and tests are carried out by manufacturers, and the process is closely controlled, so there’s little opportunity to cheat in the actual testing.

Computer-linked test cars are run up to speed on a test track; and during their deceleration to a standstill a mass of data is recorded, to calculate the energy needed to drive the car at given speeds.

This data is transferred to a programmed rolling road dynamometer, where tests are conducted either by skilled drivers, or more usually by a robot operating the car’s primary controls, and programmed to perform the test cycles.

Either method must meet tight limits of speed, acceleration, and deceleration, and select specified gears at given speeds according to declared figures regarding the engine performance characteristics.

Test cars must have been run-in for at least 3,000 kilometres, and pre-conditioning in special chambers to temperatures between 20 and 30°C is required before testing. The test car is then first run through the one kilometre Urban Cycle, working against the car’s aerodynamic and rolling resistance, as derived from the test track run.

It involves acceleration (specified in kph) up to 9.3mph, 18.6mph and 31mph, each time decelerating and stopping for a period at idle, with this basic cycle being repeated four times, achieving an average speed of 12mph.

The warm car then immediately commences the 7km Extra Urban Cycle, involving accelerations to 43mph, slowing to 31mph and further accelerations in stages up to 75mph, before decelerating to rest, with an average speed of just 39mph.

The fuel consumption, carbon dioxide and emissions like particulates and oxides of nitrogen, NOx, are concurrently measured for emissions compliance, and the Combined EC figure is calculated from the Urban and Extra Urban consumptions.

These EC tests have not significantly changed in 30 years and the acceleration rates (30 seconds plus for 0-40mph) and modest speeds of the Extra Urban test are particularly unrelated to today’s open road and motorway driving.

In contrast, America’s EPA (Environmental Protection Agency) uses an average speed of 48mph for their “Highway” test cycle, which obtains a realistic 50.4 miles per Imperial gallon figure for a manual Volkswagen Jetta 2.0 TDI that records 68.9mpg in the EC Extra Urban test.

The EPA also offers other test cycles, for “High Speed”, “Air Conditioning” and “Cold Temperature”, allowing US owners to select figures appropriate to their driving habits. Roads, traffic and driving speeds vary enormously in Britain, and achievable real life economy at best lies somewhere between the Urban and Combined test figures.

It has long been suggested that, while the EC test methods are strictly enforced, there is considerable scope to ‘optimise’ engine settings and gearing to achieve better figures, while actually delivering little or no economy gains in real life motoring.

Also, what is often forgotten is that the tests are carried out on perfectly level ground and with no deviation from straight ahead, and no correction factors are offered for those who live in hilly and/or rural areas.

Nor does the EC test ‘cold start’ with an engine warmed to 30°C compared with most people’s morning start. New EC and world test cycles have been discussed for several years, but there appears to be no immediate prospect of more realistic guides to fuel economy, while the situation is similarly unsatisfactory on official testing of the range and fuel economy of electric cars.

Doc Diesel



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