There is a just a penny between the wholesale price of diesel and petrol. But at the pumps, motorists running on diesel are being forced to pay extra to fill up their tanks.
Last Friday, for instance, the wholesale price of unleaded petrol stood at 103.5p a litre, while diesel was only slightly more expensive at 104p a litre. Yet motorists were charged a massive 6p extra.
Now the RAC is demanding that fuel retailers cut the extra charges or justify them.
The motoring organisation says retailers usually make around 4p per litre profit on both fuels. However they're now making a profit of 7.5p a litre on diesel compared to just 3p a litre on petrol, the RAC says. This works out as an extra £1.92 on the average 55 litre tank of diesel.
Businesses are being worst hit because, although two-thirds of Britain's 29 million cars run on petrol, vans and company cars use diesel to cover high mileages.
RAC fuel spokesman Simon Williams said: "In the last year retailers have been noticeably quicker to pass on reductions in the wholesale price to forecourts, but we are now seeing an unfortunate blip in that trend, where diesel prices are higher than they really need to be.
"There has been talk of the fuel retailers using higher diesel prices to keep petrol prices lower, but whether or not that's the case, the simplest way to operate must be to make sure retail prices always reflect wholesale prices proportionately.
"This has to be fairer in the long run for both private motorists and businesses alike."
Despite the gap, unleaded is now 4p a litre cheaper than it was a year ago, as retailers passed on savings in the wholesale price, which eased in mid-January to around 106p – a result of the pound's strong performance against the dollar.
Looking ahead, the wholesale price of diesel is likely to remain low in coming weeks due to the start of the American holiday season. The US has far more petrol vehicles than diesel and petrol production will be boosted to meet demand – causing a slight increase in petrol retail prices, which in turn narrows the gap with diesel pump prices.
Mr Williams said: "As we regularly see prices go up as a result of global demand issues it seems only fair that this annual drop in worldwide wholesale diesel prices caused by this little-known US petrol peak phenomenon should be reflected on UK forecourts.
"Over the years we have been encouraged to choose diesel vehicles for environmental and fuel economy reasons, which means they are the preferred option for many.
"Unfortunately, UK motorists now pay far more relatively for diesel than petrol because UK refineries are not geared up to meet demand and we have to import supplies from Russia and other countries.
"In the interests of fair play it would appear a good moment for our driver-friendly supermarket fuel stations or the big fuel companies to take a stand and cut the price of diesel, in line with wholesale prices, and give diesel car owners and business drivers a welcome respite."