David Knight, managing director of PHH Vehicle Management Services, which operates more than 200,000 vehicles, says fleets are being kept in the dark over the 'real' fuel efficiency of many models. Consequently, they may be operating cars which are not the most fuel efficient in their respective sectors.

The controversy has erupted following the existence of two different, but 'official' methods for measuring fuel consumption. Traditionally car makers have issued fuel figures based on steady speed cycles of 56mph, 75mph and a simulated urban cycle. However, a new European Commission system of measuring, which is claimed to be more accurate, includes a cold start, acceleration, deceleration and idling along with constant speed driving, and also takes account of options such as air conditioning and their effect on fuel consumption.

The EC system has been law for new engines since January 1996, but does not become law for existing engines until 1998. As a result manufacturers are issuing figures complying with the new format for new models, but with only a minority publishing both new and old mpg figures it is all but impossible for fleet operators to compare like with like.

PHH is writing to car makers who don't publish all the information calling on them to either adopt the new format for all models or publish fuel consumption figures using the old yardstick where they aren't currently doing so.