Supermarkets across the UK have lowered their petrol prices following a fall in the wholesale cost of fuel.
Asda was the first to announce it would cut up to 2p off a litre of petrol, saying customers would pay no more than 133.7p.
The AA broadly welcomed the move, but urged other retailers to do the same.
"Unless the rest of the market reflects the lower cost, it's a case of the same old story - prices up like a rocket, falling like a feather," said AA's head of public affairs Paul Watters.
The roadside recovery group is in the process of providing information to the Office of Fair Trading, which is investigating whether a fall in oil prices is being passed on to motorists.
"Last week, our fuel price report pointed to a 4p drop in petrol wholesale prices working its way through the system," Mr Watters said.
"UK average petrol pump prices reached a late summer high of around 140p a litre in mid-September and sat there for more than a fortnight.
"More than a month on, the average petrol price yesterday was down to only 138.70p a litre."
Asda cut its petrol prices by 3p a litre at the end of September and other retailers said they would follow suit.
Meanwhile, average diesel prices have fallen by 1p a litre - almost exactly reflecting the late summer fall in diesel wholesale costs, according to the AA.
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