The value of diesel fell by nearly 12p final month - a report drop, based on new figures.
The RAC mentioned the typical value per litre was now 147p, down from 158.9p initially of May - making it the seventh consecutive month costs have fallen.
It means the price of a typical 55-litre tank is round £6.50 cheaper.
However, the RAC mentioned the autumn must be higher to completely replicate adjustments within the wholesale market.
Petrol costs additionally fell by round 3p in May, from 146.5p to 143.3p.
The Competition and Markets Authority mentioned in May that rising costs in 2022 appeared to partially replicate "some weakening of competition".
RAC gas spokesman Simon Williams mentioned: "After calling for giant pump value cuts for weeks, we have been happy to see that May was the month when this lastly occurred.
"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 discount within the value of diesel in a single month is one thing we've not seen in practically 23 years of monitoring costs.
"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 factors to a extra clear and aggressive gas market there, one thing drivers in the remainder of the UK would very very like to see, significantly with cash being so tight within the cost-of-living disaster."
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Fuel costs started to extend initially of 2022, partly pushed by pressures in the marketplace introduced by the conflict in Ukraine and sanctions on Russian corporations.
UK inflation has eased in current weeks, helped by falling vitality costs, however meals costs stay excessive.
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