A DoT spokesman said the formula was rejected as a means of converting old figures to new figures en masse after a minority of motor manufacturers objected, but said the formula was adequate for making approximate comparisons between cars.
Fleet consultant David Henley Systems obtained the formula in response to calls from the industry for a level playing field (Fleet News June 21 and August 16). The problem arose with the introduction of the new fuel consumption criteria for all new cars from January this year. Most manufacturers are either working on one set of figures or the other, but only a minority publish both - making it impossible for fleet managers to make accurate fuel efficiency comparisons.
The formula requires litres/100km figures to be used. For petrol models, multiply the urban figure by 0.56, the 90km/h figure by 0.23 and the 120km/h figure by 0.22, and add the three results together to reach the new EU combined cycle figure. For diesel models, substitute the fractions 0.53, 0.23 and 0.24 at urban, 90km/h and 120km/h respectively.