Fuel price rises push up cost of full tank by around 4
Motorists faced a "shock" in August after fuel price rises that were among the biggest in more than two decades, the RAC has said.
Petrol prices rose nearly 7p per litre, pushing up the cost of filling a tank by 3.74.
Diesel went up 8p, making filling a tank cost 4.41 more, the motoring organisation said.
But the RAC, representing drivers, said fuel retailers were not increasing the profit they were making.
"August was a big shock to drivers as they had grown used to seeing far lower prices than last summer's record highs," said the RAC's fuel spokesman Simon Williams.
But he said a recent investigation by the competition regulator appeared to be making retailers more cautious about raising prices further than necessary.
"They have clearly been influenced by the Competition and Markets Authority's investigation as, all of a sudden, margins are once again closer to their longer-term averages," Mr Williams said.