Fuel Consumption

hd fuel pumpWhy you can’t trust official figures quoted by the manufacturers

When a new model is developed the manufacturer must put it through a complex series of European ‘type approval’ tests before it can be offered for sale. One of these involves the car being driven over a standard driving cycle on a rolling road to determine official fuel consumption and CO2 emissions data.

The test procedure and driving cycle – accelerations, gear changes, idling etc. – result in three official fuel consumption figures: official urban, extra-urban and combined, which is a weighted average of the other two.

Car manufacturers have to quote these in adverts, websites, brochures and so on and are not allowed to publish figures obtained by alternative means. The official CO2 emissions figure determined in the test goes onto the registration document and is used to determine the applicable rate of car tax.

Divorced from reality

A recent study by independent environmental think tank ICCT showed that on average drivers were achieving 21% worse fuel consumption than expected from the official figures.

Back in 2001, the difference between official figures and ‘real world’ was a more reasonable 8%.

By way of an example this means that a car with a claimed official combined economy of 45mpg might on average have achieved around 41mpg in 2001 but only around 35mpg today.

In fact the discrepancy could be even worse. Research from Holland has suggested that cars use 35% more fuel than official figures indicate. This Dutch study also showed that the more economical a car is to run, the greater the gap between official and real world fuel consumption. This means that the official figures can neither be relied on to indicate absolute fuel consumption or fuel consumption relative to another model.

Driving cycles

motoring_feature_fuelNo single test can ever tell you exactly what fuel economy you will get thanks to the way driving style, weather and traffic conditions all effect fuel consumption, but the test should be broadly representative.

The current fuel consumption test uses a basic driving cycle first developed in the 1970s and, though it has evolved, it is quite short and features a lot of idling, slow acceleration and low engine load. The test car only briefly reaches motorway speed.

Air conditioning and other electrical loads are turned off and the test takes place at a relatively high 20-30C ambient temperature.

Additionally, many flexibilities and tolerances in the test procedure can be exploited to give a lower fuel consumption figure. A new, more relevant test cycle is being developed along with more tightly controlled test procedures but it will be some years before these become adopted into European legislation.

Rule of thumb

If you’re buying a car up to say five years old then you shouldn’t be disappointed if you work on the basis that you will achieve around 25% worse than the official combined fuel consumption figure quoted.

For older cars 10% worse than the official combined figure is probably a reasonable expectation.

Fuel efficient driving tips

A new campaign backed by the AA and five other leading automotive industry bodies claims that official European mpg (fuel efficiency) figures are within every driver’s reach just by adopting simple improvements to their driving style. Motorists could save around 20% on fuel costs by making a few simple changes to the way they drive.

Fuel efficient driving tips to try:

  • Read the road – helps reduce harsh acceleration and braking
  • Reduce drag – removing bike racks when not in use or keeping windows closed at high speed helps cut fuel waste
  • Keep tyres inflated – the industry estimates that if all tyres in use were correctly inflated we’d save £440 million in fuel costs every year
  • Remove excess weight – taking out unnecessary items is a cheap and easy way to save fuel
  • Be energy smart – opening windows at lower speeds can be better for fuel efficiency than using airconditioning at low speeds
  • Consider a driving efficiency course – investing in a driving efficiency course could pay for itself in the savings you’ll make
  • Regularly service your car – simple maintenance can make an important difference to fuel consumption
  • Buy a newer vehicle – according to official figures, today’s cars are around 23% more efficient than they were just 10 years ago
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