Drivers faced different fortunes at the pumps in May, as diesel prices fell 4.5p a litre while petrol rose 2.4p, new RAC data shows.
The average price per litre of unleaded increased from 157p to 159.4p, adding £1.32 to the cost of filling up a typical 55-litre family car. Diesel, meanwhile, fell from 188.4p to 183.9p, saving drivers £2.50 a tank.
The price differences were caused by the wholesale cost of petrol rising mid-month while the diesel equivalent reduced, the RAC said, closing the gap between their wholesale prices to 10p at one point.
On 14 May, a litre of delivered petrol cost retailers 123.5p – before retailer margin and VAT – while the diesel equivalent was 133.3p, according to the motoring services company. It added that the current gap between pump prices for the two fuels was almost 25p.
The RAC said buying petrol at one of the big four supermarkets worked out to be 4p a litre cheaper in May as the average price was 155.6p at the end of month, compared to the UK average of 159.4p.
Supermarket unleaded went up 1.4p in contrast to the UK average increase of 2.4p. Supermarket diesel, however, was only 1.4p less than the UK average at 182.5p on 31 May, having come down 3.9p in the month.
“It’s really quite unusual to see the price of one fuel go up and the other come down,” said RAC head of policy Simon Williams.
“This was brought about by the wholesale cost of unleaded rising due to higher demand in the United States. This coincided with a drop in demand for heating oil in western Europe which is refined from the same part of the barrel as diesel.
“The last week of the month also saw the price of Brent crude drop below $100, which ought to prove to be good news for drivers as it should lead to far lower prices. The wholesale price of diesel has been far lower than its peak for some time which we had hoped would yield far bigger forecourt reductions.
“Petrol, however, could have risen further than it did, so it might have been the case that retailers decided to keep the price of unleaded down at the expense of greater diesel reductions.”
On 1 June, the CMA found soaring oil costs remained the key driver of pump price increases since the Iran war, with no evidence of retailers actively changing their pricing strategies to take advantage of the crisis during March and April.
It followed investigations into fuel margins between February and March, which on average remained “broadly unchanged” at 10.3ppl and 10.7ppl respectively.
The CMA will publish its next update in August, which will look at market developments over a longer time period until the end of June.
“We hope its next report shows that drivers have been treated fairly by retailers reducing their pump prices in line with the fall in wholesale costs,” added Williams.
The government officially launched its Fuel Finder scheme in February, requiring all UK forecourts to report changes to pump prices in real time to boost transparency and competition.
Under the Motor Fuel Price (Open Data) Regulations 2025, the CMA has the statutory role of enforcing compliance by forecourt retailers. Where potential breaches are referred to the CMA by the scheme’s aggregator VE3 Global, such as failing to report accurate data, the CMA has powers to investigate and take enforcement action, including imposing fines.
closing the gap between their wholesale prices to just 10p at one point.
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