New car road tax guideTesting emissions and efficiency

Cars are tested under highly controlled conditions

Cars are tested on a ‘rolling road’ in a laboratory, under highly controlled conditions. Exhaust gas emissions are measured, rather than actual fuel flow, and the fuel consumption is calculated from this.

Testers follow a set series of driving sequences that are rigorously controlled, and in which they have to use set gears and control the throttle position to keep a performance ‘marker’ between a high and low point. It is a fine balancing act.

An ‘urban’ test is conducted on an engine started from cold, with a maximum speed of 31mph and a minimum of 12mph. The total distance measured is 2.5 miles. An ‘extra-urban’ element then takes place over 4.3 miles, and includes faster speeds between 39mph and 75mph.

The ‘combined’ figure, which car makers quote when promoting new cars, takes a weighted average of these two results.

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