Fleets are being stung by the massive difference between the price of diesel and petrol being charged by the country’s retailers.

The difference in pump prices between a litre of diesel and a litre of petrol has dramatically increased over the last 12 months.

According to Arval, the price difference between the two fuel types in November 2007 was 2.6 pence per litre, but by July 2008, when the price of a barrel of oil was at its highest ($147), the difference had risen to 14 pence per litre.

However, while the price of a barrel of oil has now returned to less than $50, the difference between the cost of the two fuels at the pumps has remained at a level more than three times that of a year ago.

This week, a litre of diesel will cost fleets £1.05, while a litre of unleaded will be considerably less at 89.6ppl.

The Petrol Retailers Association (PRA) said the difference between the cost of petrol and diesel is because the UK has “less refining capacity for diesel than petrol”.

Hence it will cost more.

Demand for diesel grew rapidly a decade ago when many fleets switched fuels to take advantage of diesel’s better economy.

Now close to 100 per cent of fleets specify diesel cars and, across the board, diesel sales are almost at parity with petrol.

However, the PRA says the country’s refineries have not yet caught up with this change in demand.

“The creeping gap between the price of a litre of petrol and that of diesel appears to have passed most people by.

"There’s no single explanation why this is but it doesn’t mean that diesel car drivers can’t do something about reducing their monthly fuel outlay,” said Paul Busby, Paramount Performance’s MD.

“A properly tuned diesel engine gives drivers more miles in the tank and erodes the price differential that diesel drivers are suffering from.”