UNDER the current regime, official fuel consumption figures in the EU - which are the ones relied upon by manufacturers to market cars - are calculated by in laboratory conditions, with vehicles placed on a rolling road and the ambient temperature set between 20C to 30C, writes William Scholes.
As the chart on the facing page makes clear, the integrity of the test is open to all sorts of manipulation. It is also hopelessly outdated and inadequate.
Three different fuel consumption figures are recorded: urban; extra-urban; and combined.
The urban test cycle is carried out from a cold start and is supposed to replicate town or city stop-start driving, meaning the car is accelerated, decelerated, held at a steady speed and allowed to idle. The car isn’t allowed to exceed 31mph, must have an average speed of 12mph and covers just 2.5 miles.
The extra-urban figure is recorded immediately after the urban cycle test, meaning the engine is warmed through. Half the test involves driving at a constant speed, with accelerations, decelerations and some idling making up the rest. The top speed is 75mph and the average is 39mph, with a distance covered of 4.3 miles.
The combined figure takes the average of urban and extra-urban tests, weighted according to the distances covered in each.
CO2 emissions are recorded during the urban and extra-urban part of the fuel consumption test and a combined figure worked out. It is this figure that is quoted as a particular vehicle's CO2 output, which is used to calculate how much tax it is liable for.
The combined figure is generally regarded as the most representative of every-day driving, but as you can see from the way the fuel consumption is calculated as well as the flexibilities exploited by manufacturers, it’s little surprise that they fail to match up with the reality of how often you visit the filling station forecourt.
If the gap between your real world MPG and the official figure is woefully large - and costly - then you can be sure that your car's CO2 emissions are also greater.
Where possible in Drive, we seek to give a ‘real world’ fuel consumption figure calculated on the basis of a car’s test period with The Irish News.
Despite its recent bad publicity, diesel is still unbeatable when it comes to achieving superior real world fuel consumption.
The list of the most economical ‘real world’ cars tested by The Irish News, each breaking the 60mpg barrier is headed by the Renault Clio dCi 90; its real world 62.7mpg is 71 per cent of its official EU combined 88.3mpg.
Other strong performers include: Honda Civic 1.6 i-DTEC (61.7mpg/78.6 per cent/78.5mpg); Seat Leon ST 1.6TDI Ecomotive (60.8mpg/71 per cent/85.6mpg); and BMW 116d EfficientDynamics (60.1mpg/81 per cent/74.3mpg).
Don't expect some of the new breed of smaller, so-called downsized, petrol engines to deliver their promised fuel savings in the real world.
Ford's 1.0-litre EcoBoost engine in 123bhp tune just about manages half of its official figure in a Focus (33.5mpg/56 per cent/60.1mpg), while Fiat's 0.9-litre TwinAir unit, as plumbed into the Panda, is even worse (31.9mpg/55 per cent/57.6mpg).
