The problem lies in the potential discrepancy between the figures published in the Vehicle Certification Agency's official 'New Car Fuel Consumption and Emission Figures' booklet, and the actual figure which appears on a vehicle's registration document. Different tyres are enough to alter the CO2 rating of even the most common car.
The Ford Focus 1.4i hatchback, for example, emits an average of 153g/km of CO2 with 175/70 R14 tyres, 155g/km with 185/65 R14 tyres, and 160g/km with 195/55 R15 tyres. All three variants feature in the VCA booklet, but fleet managers would be hard-pressed to know which tyres would be fitted until the vehicle arrives, and by that stage it would be too late, because the car's CO2 rating would be indelibly stamped on the vehicle's registration document.
Yet a difference of just 1g/km of CO2 could see a car move up from one band to another in the proposed four-band VED system. This could then inflate a fleet's road fund licence budget by hundreds of pounds per vehicle over a typical three-year holding period. An additional 1g/km of CO2 could move a company car into the category above in the proposed benefit-in-kind tax regime, thereby increasing the percentage of its list price on which the driver pays tax.
Stewart Whyte, managing director of Fleet Audits, said: 'If you order a car based on a VCA booklet reading of 199g/km of CO2, but the specification changes in the meantime and the car's V5 shows 200g/km, do you have a claim?' He has been lobbying the Government to make the VCA booklet more user-friendly by listing actual models, such as a Ford Focus 1.4 LX five-door manual, rather than the highly technical description that currently appears, such as 'Focus 1.4i 16V saloon <1090kg unl.mass (175/70 R14 tyre)'.