Consumption claims don't reflect reality
Fuel consumption figures quoted for new cars are often alluring, especially for hybrid models fitted with a petrol or diesel engine mated to an electric motor.
Frugality is a key issue for today's customers at a time when fuel costs are soaring, say experts at the Automotive Research (CAR) centre based at the University of Duisburg-Essen in Munich, Germany. Yet many of the figures used by manufacturers to promote their vehicles can mislead consumers.
This is partly due to outdated measurement norm and testing cycle results, which look good on paper but have little in common with driving reality. As a result, certified values are often not achieved in practice, say critics.
A recent test by Germany's ADAC motoring club, the country's largest, showed that BMW's ActiveHybrid 3 Sport Line Automatic used an average of seven litres of petrol per 100km - 19 per cent more than the usage claimed by the maker. The Audi A6 2-litre TFSI Hybrid Tiptronic acquitted itself badly in mid-2012 with 6.9 litres per 100km, or 11 per cent more than the official figure.
The eyebrow-raising results are not just confined to hybrids. Across the board and regardless of motive power, cars are using more fuel than the makers claim in their technical specification sheets. An ADAC test series in 2012 showed negative variations of up to 30 per cent. Only one of eight cars put through their paces delivered fuel consumption results which tallied with those given.
Manufacturers' figures for the power hunger of electro-mobiles are also unreliable, the ADAC observed. One Swedish model consumed a hefty 28.3 kilowatt hours in daily use - nearby double the specified 14.7kw/h rating.
Experts blame the New European Driving Cycle (NEDC) for the results which show many cars failing to live up to their expectations.
The NEDC is designed to represent the typical fuel usage of a car in Europe over a fixed distance. The test procedure consists of repeated cycles of urban driving together with what is known as an Extra-Urban Driving cycle (EUDC). This represents more aggressive driving at higher speeds.
The stylised tests are carried out on a roller bench which uses electricity to mimic aerodynamic resistance and overcoming inertia. Humidity, air pressure and temperature are set beforehand.
According to the German environmental aid organisation Bundesumwelthilfe, carmakers often equip the test vehicles with lightweight alloy wheels shod with tyres with lower rolling resistance.
High-tech motor oils reduce engine friction and the battery is also charged up fully beforehand to cut the power sap from on-board ancillaries and gadgets. Sometimes even the alternator - which recharges the battery when the engine is running - is disconnected for the duration of the test.
Hybrid vehicles can only run with fuel-saving electric assistance for as long as their batteries hold out and the NEDC cannot hope to simulate everyday driving conditions, according to Axel Knoefel, who works as a test engineer at an ADAC technical centre. The batteries of a hybrid are charged up to the limit beforehand and any electric running is unrealistically classified as zero consumption.
Electric-only vehicles are treated in much the same way. Makers have a whole box of technical tricks with which to massage the figures.
Knoefel said that one of the tests - the EC 101 test which is one of the European Union test regulations - was conducted at a constant 22C. This is an ideal temperature for batteries which function better when conditions are warmer. In wintertime owners can expect range to go down by 30 per cent as the batteries battle with the cold.
From 2017 all cars and light commercial vehicles will be subject to a new, more realistic procedure known as the Worldwide Harmonised Light Duty Test Procedure (WLTP).