12 Aug 08
The official tests
Official fuel consumption tests have been in use since the 70s, but the current procedure has followed a Europe-wide standard since 2001.
The tests are carried out in a laboratory on cars which have been run-in and driven for at least 1,800 miles. Urban and an extra-urban cycle are tested, with a combined fuel consumption figure then calculated as an average of the two. Exhaust emissions are also measured.
Independent test organisations, manufacturers or importers carry out the tests at their own facilities, but under stringent European Commission directives in order to achieve consistency.
Before the results are officially recognised in the UK, the Department for Transport inspects the test laboratories and witnesses some tests being carried out, or checks that the figures have been certified by a European government.
Our tests
Our comprehensive set of tests embraced a broader spectrum of real-life driving styles and operating conditions. We put the cars through five test cycles, rather just two - and crucially, these were all on-road, with accurate measurement of fuel consumed, rather than analysis of the exhaust in a laboratory.
The five test cycles were carried out with specialist fuel-measuring instrumentation, careful control of weight and fixed rates of acceleration and deceleration.
To iron out any inconsistencies in driving style, each car took the same route in the early morning to avoid traffic. All stops and starts were carried out in the same places and each test was carried in dry conditions without significant wind speeds. The car's air-conditioning and electrical items such as heated windscreens were switched off, as in the official tests.
So, for example, our Urban Test took 11.3 minutes, at an average speed of 17mph; it comprises 12 stops, plus three 'street corners' at 10mph, as well as a brief period at 30-40 mph. The car was stationary for 180 seconds in total, 26.5% of the time.
Our average or 'overall consumption' figure was then calculated by combining 10% Urban, 15% Suburban and 25% each for the other three - Motorway, Brisk/Out-of town and Gentle/Rural driving.
Three modes - Suburban, Brisk and Gentle - include a cold start from a realistic (for the UK) 10degrees C.
