Diesel costs 14.2 pence per litre more.
For an average fleet of 100 diesel-powered cars, each of which covers 20,000 miles a year, the additional cost over 2008 at present pump prices compared to just 12 months ago is £32,400, or an extra £324 per car.
The rises have been attributed to the price of crude oil, which peaked briefly at $100 a barrel at the start of year, prompting fears of further rises at the pumps.
“High-priced oil is here to stay, and companies need to ensure that their fleet and travel strategies are able to cope,” said Mark Sinclair, head of Alphabet.
“Even though oil prices will almost certainly fall back into double digits for a while, the trend for 2008 will be for much more costly oil.
“Although this will drive up business costs across the board, fleet will be the key area where firms can directly mitigate the impact on the bottom line.”
As the new year arrived, the average price of unleaded petrol hit 103.3 pence per litre – a record that is predicted to be shattered in the coming days, according to the AA.
Diesel prices fared a little better at 107.95 pence per litre, which remains marginally below the record of 108.00 pence per litre, set on December 6, 2007.
“Vehicle acquisition policy will be critical, because today’s purchasing decisions lock in fuel consumption and CO2 emissions for typically three years ahead,” said Mr Sinclair.
“Maximising the fuel efficiency of all vehicles used on business will be a vital element of management.”
Additional bad news to the expected rises in the coming days will be a two pence per litre fuel duty increase that will come into effect in April.