The cost of filling up an average diesel car has dropped by £3 in a month as the average price of diesel at the pumps fell 5p a litre, data shows.
The fall - hailed as good news for people setting off on holidays - came as retailers began passing on the savings in the cost of wholesale diesel, which has been below that of petrol since the end of May.
At the start of the month diesel was 120.63p, but by the end it had dropped almost 5p to 115.74p, according to the RAC's Fuel Watch data for July.
And on Wednesday July 29 the country saw the first forecourt price flip between diesel and petrol since summer 2001 with diesel at 116.28p, just below the average petrol price of 116.64p, the RAC said.
Half a penny a litre also came off the petrol price in the month, which reduced from 117.05p to 116.51p.
Both petrol and diesel wholesale prices remain low as a result of the price of oil falling below the 60 US dollars-a-barrel mark on July 3 and remaining there ever since.
In the course of the month, the oil price fell 9% from 61 US dollars to 52 US dollars a barrel.
This - combined with the pound staying strong against the dollar at 1.56 US dollars - is keeping petrol and diesel cheaper, the RAC said.
RAC fuel spokesman Simon Williams said July was "a month of good news for motorists with diesel vehicles".
He added: "The 5p-a-litre diesel saving recorded in July means the cost of filling up an average 55-litre diesel family car, such as a Ford Focus, has dropped by £3 in a month. This is particularly welcome for motorists setting out on their annual holidays who are driving long distances to destinations in the UK.
"And, if the supermarkets continue to battle over prices as they pass on further wholesale cost savings, pump prices should reduce to around an average of 111p a litre - a price last seen in January 2010.
"This would shave another £2 off a tank of diesel. To put this in perspective, this is even lower than the 113p average price reached in January after oil hit a near six-year low of 45 US dollars a barrel.
"The cheaper wholesale price of diesel - now almost 6p a litre cheaper than petrol - has been brought about by the fact two new refineries in Saudi Arabia are now producing diesel to meet the large European demand.
"We expect this to be good long-term news for the nation's 10.7 million diesel car drivers as well as for businesses operating commercial vehicles. Everyone should benefit from a better, fairer deal at the pumps going forwards."
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