Information in this article refers to EU fuel consumption testing regulations.
When car manufacturers advertise their vehicles they include details of the expected fuel economy for urban, extra urban and combined cycles. This article explains the difference between these figures, why you'll probably be unable to achieve them through everyday use of the vehicle, and why they're still really useful when you're thinking about what car to buy.
The information in this article applies to UK regulations, but other countries will follow a similar process.
The fuel economy figures are the number of miles the car will travel per gallon of fuel. An imperial (UK) gallon is around 4.55 litres. There are three types of figures that are commonly used when presenting this information in marketing material:
- Urban: Car is tested over a distance of 2.5 miles at an average speed of 12mph. The cycle consists of a series of accelerations, constant speed sections, decelerations and idling.
- Extra Urban: Immediately follows the Urban test so the engine has warmed up. Similar to the previous test but over 4.3 miles with an average speed of 39mph.
- Combined: The average result of the two previous tests, weighted by the distance covered in each. The Extra Urban test provides around 63% of the score and the Urban test provides the remaining 37%.
Real world results
It's not likely that you will achieve the same fuel consumption reported by the manufacturers in these tests. This is because the tests are carried out in a laboratory with a controlled environment. Manufacturers are allowed to make certain modifications to the car to remove as many variables as possible that may affect fuel efficiency. These modifications include:
- Taping up gaps between panels in the vehicle, such as around the doors.
- Removing wing mirrors to reduce drag.
- Disconnecting electrical systems.
- Disabling engine components that are not required.
- Over-inflating the tyres to minimise friction.
Manufacturers aren't necessarily making these changes with an intention to trick anyone. The emissions tests for CO2 are usually carried out at the same time as the fuel consumption tests, and ensuring the engine uses less fuel will reduce the final emissions rating. This means the manufacturer can sell the car with a lower tax band and customers will pay less road tax.
When you're driving on the open road many additional factors make a difference to the fuel economy of your car. Strong winds, open windows, weight of passengers and using electrical devices such as the heater, radio or air conditioning can all significantly increase the amount of fuel your car uses. When you also take into account the driving styles of different motorists the results can be very different.
Even though the results of the fuel economy tests don't apply in the real world, they can be used to compare the consumption of cars you might consider buying. This is because they're all relative - all the manufacturers are bound by the same regulations and requirements for the tests.
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