Are official fuel economy figures accurate?

March 25, 2011 by Perrys Content Team in Buyers Guides

It is not uncommon for small, lightweight city cars to boast official fuel consumption figures above 80mpg. For the average driver though this figure will not representative of what is actually happening out on the road.

Fuel economy and how it affects motoring fuel costs in Britain is a hot topic. Here we discuss what the official fuel economy figure represents, why it is important but also why it also shouldn’t be used as a definitive measurement of how much you will spend on fuel.

What is the fuel economy figure?

The official fuel consumption figure of every car is a number showing how many miles the car will be able to travel on one gallon of fuel – using the term miles per gallon or ‘mpg’.

The figure is often referred to as fuel economy because it shows how much, or how little, fuel is used over a certain distance. The fuel economy figure is a way of showing how efficient the car is in terms of fuel used.

As expected, a higher figure will mean less fuel is used per mile and will subsequently mean fewer trips to the pumps for the driver.

Generally, the more frugal a car is in terms of fuel consumption, the better the CO2 emissions figures are because less fuel is used. This has benefits in terms of Vehicle Excise Duty (VED), congestion charge and company car tax – since all are based on CO2 emissions.

How are official fuel economy figures calculated?

Understanding how the fuel economy figure is calculated goes a long way to explaining how the numbers contrast with real life fuel consumption.

Before going on sale across Europe, each new car and every engine option for that car is independently assessed in the European Government’s standard test. This is known as the New European Driving Cycle (NEDC).

The assessment takes the form of two tests. These are for urban and extra-urban figures. You may have seen these figures quoted when buying a car, but realistically, only the combined cycle consumption figure should be used.

This is an average of both tests undertaken in a laboratory on a rolling road. The first, the urban test, involves the car being subjected to a rolling road. Although speeds never go higher than 31mpg, and average 12mph, the test features a series of accelerations, decelerations, idling and steady driving to replicate driving in busy city traffic.

The urban figure is generally the highest mpg figure. However, the car is then tested in a similar way but at higher speeds (averaging 39mph) for the same distance.

This will result in a much lower figure and is called the extra-urban cycle. An average is then taken of the two to find the combined cycle fuel economy figure.

The combined fuel economy figure calculated for a new car is the type most often cited by manufacturers when disclosing details.

How can I work out my car’s true fuel economy?

If you want to check if your car is matching its official fuel economy figure then you might be able to do easily via the dashboard. Some modern cars display the current fuel economy figure being achieved among other information on the dashboard display screen.

If this isn’t possible in your car then the fuel economy can be calculated by filling the tank and making a note of the mileage. Then divide the amount of fuel used by the number of miles covered.

However, since the fuel pumps in Britain give readouts in litres only, you must convert litres to gallons before working out your car’s mpg. Divide the number of miles covered by the amount of litres and multiply the answer by 4.546 to get the final mpg figure.

Why can’t I achieve the combined cycle fuel economy figure?

In theory, taking an eco-friendly approach to your driving should allow you to achieve a figure near or even above the official fuel consumption number.

However, the fact the tests are conducted in laboratory conditions at steady temperatures between 20 and 30 degrees Celsius, makes it more difficult to recreate them in the real world.

Extra weight from luggage or passengers, wind and other weather conditions, poor road surfaces and other real world variables will mean more fuel is inevitably used in the real world than in laboratory tests.

Is the official fuel economy figure useful?

With careful driving, you shouldn’t be too far away from the official figure, and in this way the numbers can be used as a loose guide to what you should be aiming for if your aim is to save on fuel costs.

It is also useful when you are looking to buy a new car. The same test is completed on every new car, and the mpg figure becomes a useful comparison tool when comparing the environmental effects – and the effects on your bank balance – of a potential new model.

What cars have the best fuel economy?

Generally small, lightweight cars with smaller engines will offer better fuel economy than larger cars and more powerful engines.

Although not a definitive fact, diesel engines usually offer better fuel economy than similar-size petrol engines, but diesel currently costs 10p per litre more at the pump, so it is up to the individual to decide if the extra outlay is worth it.

Some manufacturers highlight their most efficient cars with an eco-signature. Popular examples include Vauxhall’s ecoFLEX.

What do you think? Leave a comment.