The price of diesel fell by almost 12p last month – a record drop, according to new figures.
The RAC said the average price per litre was now 147p, down from 158.9p at the start of May – making it the seventh consecutive month prices have fallen.
It means the cost of a typical 55-litre tank is around £6.50 cheaper.
However, the RAC said the fall should be greater to fully reflect changes in the wholesale market.
Petrol prices also fell by around 3p in May, from 146.5p to 143.3p.
The Competition and Markets Authority said in May that rising prices in 2022 appeared to partly reflect “some weakening of competition”.
RAC fuel spokesman Simon Williams said: “After calling for big pump price cuts for weeks, we were pleased to see that May was the month when this finally happened.
“The fact it appears to have been prompted by the Competition and Markets Authority’s mid-month announcement about weakening competition in fuel retailing is surely not a coincidence.
“A 12p reduction in the price of diesel in one month is something we haven’t seen in nearly 23 years of monitoring prices.
“But despite this, it’s still galling to see that a litre of diesel is 8.5p cheaper in Northern Ireland than it is in the rest of the UK.
“This points to a more transparent and competitive fuel market there, something drivers in the rest of the UK would very much like to see, particularly with money being so tight in the cost-of-living crisis.”
Fuel prices began to increase at the start of 2022, in part driven by pressures on the market presented by the war in Ukraine and sanctions on Russian companies.
UK inflation has eased in recent weeks, helped by falling energy prices, but food prices remain high.