To allow consumers to compare one vehicle's fuel consumption against another, vehicle manufacturers are required by law to provide fuel consumption figures for the vehicles they produce. These figures are included on labels fitted to all new cars, in vehicle sales brochures and also on the Green Vehicle Guide web site.
Manufacturer supplied fuel figures are determined using a standard laboratory test that is designed to exclude the myriad of variables that a vehicle is subjected to in everyday driving. These variables include road, traffic and weather conditions, driving style and vehicle speed, load and condition. The figures produced cannot therefore be used as an indication of the fuel consumption a vehicle might achieve under actual operating conditions. Nor can they be compared to figures produced by on-road testing or on-board trip computers.
For most motorists, the only readily available fuel consumption test method involves actual on-road driving and calculation of fuel consumption based on the distance travelled and the volume of fuel used. Providing care is taken, this method offers a reasonably accurate means of determining actual fuel usage for a particular trip. However, in addition to the vehicle and road related variables already mentioned, the accuracy of service station pumps and the vehicle's odometer, the consistency of fills and even the ambient temperature and temperature of the fuel during filling can influence the final result produced by this method.
The following formula is used to calculate fuel consumption in litres / 100kms, the most commonly used measure of fuel consumption.
Example a car uses 65 litres to travel 500km
(65 X 100) / 500 = 13
(Litres used X 100) / km travelled = Litres per 100km.
It's important to note when comparing on-road fuel consumption figures that differences in road, weather and vehicle conditions can alter fuel consumption figures considerably, even for an apparently similar trip.
On-board Trip Computers
Many vehicles are fitted with a trip computer that provides an average fuel consumption function. The fuel consumption figure produced by an on-board trip computer is calculated from the distance travelled and the theoretical amount of fuel consumed by the engine (often based on the number and duration of fuel injection pulses). In theory, these consumption figures should be reasonably accurate, though in practice there are sometimes differences between these and the figures derived from on-road testing. This may be the result of variations in service station pump readings, odometer readings and filling errors mentioned above, or in the process used to calculate the trip computer figure. In view of this, these figures should be regarded as a reasonably close guide rather than a firm consumption figure.
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