UK petrol prices have for the second month running fallen by more than 4p a litre, the AA has said.
It is a reduction not seen since the price collapse of late 2008.
However, AA Fuel Price Report research shows that, in late April, diesel car owners could have enjoyed pump prices much closer to petrol's for the first time in two years.
The AA said it believed the Transport Secretary Justine Greening's calls for fuel price transparency, after an intensive AA campaign, may have helped contribute to a more rapid price fall. But the driving group also warned fuel duty is due to increase again by 3p litre on August 1.
June's AA Fuel Price Report shows that, since mid-May, average UK petrol prices have fallen 4.63p a litre, from 138.40 to 133.77.
This is on top of the 4.08p-a-litre fall the previous month and is the biggest monthly drop since the petrol price dived 5.4p a litre between mid-November and mid-December 2008.
Even so, the overall 8.71p-a-litre reduction in the pump price petrol since the 142.48p record on April 16 is still short of where it should be, the motoring group said.
Under government pressure to make pump prices transparent, the fuel industry will have to prove in the future that UK drivers are paying a fair price for fuel, the AA said.
In the past month, the average pump price of diesel has fallen 4.99p a litre, from 144.30p in mid May to 139.31 now.
But UK diesel costs at least 4p a litre more than in Europe.