Latest figures produced by Godfrey Davis Contract Hire (GDCH), the business-to-business division of Bank of Scotland Vehicle Management, show that average company car CO2 emissions have fallen by just 2.1g/km in the past 12 months.
GDCH notes 'insignificant' drops for both petrol and diesel and says this falls well short of the annual 10g/km reduction that the company car parc needs to achieve to remain tax neutral.
The GDCH CO2 Tracker measures the CO2 emissions performance of new registrations joining its 30,000-strong corporate fleet on a quarterly basis, as well as the split between petrol and diesel engines.
Nigel Underdown, head of customer relationships for Bank of Scotland Vehicle Management, said: 'More and more company car users are climbing aboard the diesel bandwagon.
'Whereas in the past this was predominantly high-mileage business drivers with lower or upper-medium sector cars, it is apparent that management ranks are now joining the exodus to diesel also.
'How much of this is the result of drivers seeking to reduce tax or down to diesels being a more acceptable proposition these days is hard to say, but the net result is that these drivers will be keeping ahead of the game in tax terms.'
The company suggests that although the industry will see more drivers making the transition to diesel, it believes that there will always be a raft of 'petrol diehards' who will resist change or in some cases, perhaps due to employer restriction, be unable to cross-over.
Underdown added: 'The indications are that the petrol sector will be left with predominantly a residue of gas-guzzlers and that the CO2 average for such vehicles may actually rise in the short-term, as we have seen in the last quarter.'
The number of diesels registered by GDCH in the last quarter rose to an 'unprecedented' 64%.