How are official fuel economy figures calculated?

Almost all car manufacturers compete for kudos and business by publishing their fuel economy figures and hoping that they are better than their competitors. Only supercar companies, where fuel economy doesn’t matter much to the owners (think Ferrari and Lamborghini), don’t pay too much attention to crowing about it. However, recent changes in European legislation has had them looking at how to bring the overall total down.

What’s a typical figure for a car these days? A quoted figure anywhere between 5-9 litres per 100 kilometres driven is typical for a small or medium sized car, and anything up to 13 litres per 100km for a larger car or SUV. But this quoted figure is not the real world figure that you will achieve when driving. It is a figure derived from a specific set of tests performed usually on a bench that simulates driving. In that sense, it’s a good way to compare apples with apples – i.e. you know that all cars are tested roughly equally – but if your new car is quoted at 5l/100km, you should probably expect closer to 7l/100km in everyday driving (and more than that if you sit in rush hour traffic a lot).

The fuel economy figure describes the how much fuel is consumed in relation to the distance traveled. The most common expressions of this are litres per 100km, kilometres per litre or (if you are in America or the UK), miles per gallon.

The European Driving Cycle is the name for the test that is performed on the engine. If it’s not done on a bench, then a professional driver will perform the test on a flat road at a temperature of around 25 degrees Celsius.

The measurements given to the public are:

• First 780 seconds of simulated driving in an urban environment (i.e. urban).
• 780 – 1180 seconds of simulated driving in an extra-urban environment (i.e. highway)
• The total of the test is called the combined cycle or complete cycle.
• CO2 emissions are calculated on the complete cycle.

Urban cycle

1. Start the engine and wait 6 seconds in neutral
2. Change into first gear and wait 5 seconds before moving off
3. Accelerate to 15kph in 4 seconds
4. Hold 15kph for 8 seconds
5. Brake to zero in 5 seconds
6. Stop for 21 seconds
7. At 49 seconds accelerate to 32kph in 12 seconds
8. Cruise at 32kph for 24s
9. Brake to a full stop in 11s
10. Pause for 21s
11. At 117s accelerate to 50kph in 26 seconds
12. Cruise at 50kph for 12s
13. Decelerate to 35kph in 8s
14. Cruise at 35kph for 13s
15. Brake to a full stop in 12s
16. Pause for 7s

This is now 195s and a theoretical distance of 1017m. The cycle is repeated another three times giving a total theoretical distance of 4067m over 780s with an average speed of 18.77kph.

For manual gearboxes, the gears are changed at specific times.

The system is not representative of motoring for the majority of people. The average vehicle performance to 100kph in 2007 was 9 seconds. These  tests were devised in the 1980s when the average acceleration to 100kph was over 14 seconds. Most modern cars will reach 50kph in 4-6 seconds, whereas point 11 sees a 26-second acceleration time. While the cycle might represent a certain amount of stop-start traffic, in general it needs to be updated to represent modern cars and driving.

Extra-urban cycle

The extra urban cycle is designed to simulate slightly more aggressive driving at higher speed. The maximum speed is limited to 120kph.

• Idle for 20s
• Accelerate to 70kph in 41s
• Cruise for 50s
• Decelerate to 50kph in 8s
• Cruise for 69s at 50kph
• Accelerate back to 70kph in 13s
• Cruise at 70kph for 50s
• Accelerate to 100kph in 35s
• Cruise for 30s
• Accelerate to 120kph in 20s
• Cruise for 10s
• Brake to a stop in 34s
• Idle for a final 20s.

Total duration is 400s and the theoretical distance is 6956m with an average speed of 62.6kph.

Should the testing regime be changed?

Some people have called for a more realistic testing regime. This would cause some serious concerns for some manufacturers in Europe though, where concessions are made for cars that achieve a CO2 output of less than 100g/km. More rigorous, real-world testing would undoubtedly see the majority of them register more than 99g/km of CO2.

A new test called the Worldwide Harmonized Light Vehicle Test Procedures is proposed to be implemented in October 2015.