Deloitte has produced a new wholelife cost calculator – Car Selector – which takes every parameter for running costs into account, including the relationship between CO2 emissions and fuel economy.

“There are some 4,500 derivatives on our system.

“So if fleet managers want to narrow down the choice before starting to use the software, we’ve found a ‘rule of thumb’ formula that works in almost all case,” explained Nigel Morris, senior manager global employer services, Deloitte.

The formula works by subtracting a car’s mpg figure from its CO2 emissions rating.

The lower the number, the lower the wholelife cost rating, enabling fleet managers to compile a league table of vehicles.

Morris said managers could use this rule of thumb to establish different choice lists on comparable cars.

After reviewing the system, CAP forecast manager Jeff Knight, said: “As a quick reference tool it works and will be useful for all those setting fleet policies.

“Anything that increases transparency for those exposed to cost and risk is to be welcomed.”