Fuel prices are still considerably lower than they were during the summer when the average cost of petrol was over 190p per litre and that of diesel was just shy of 200p.
According to the RAC, retailers have increased their profit margins, preventing motorists from receiving a further 10p reduction in gasoline costs.
The motoring organization said a further oil price fall in September pushed down the average price at forecourts by nearly 7p to 162.9p.
It was the sixth biggest monthly drop since 2000, but the RAC says it should have been bigger.
“Drivers really should have seen a far bigger drop as the wholesale price of delivered petrol was around 120p for the whole month,” said RAC fuel spokesman Simon Williams.
“This means forecourts across the country should have been displaying prices around 152p given the long-term margin on unleaded is 7p a litre.
“In stark contrast to this, RAC Fuel Watch data has shown margins to be around 17p a litre – a huge 10p more than normal.”
Supermarkets usually charge about 3.5p per litre less than the UK average but are now only about 1.5p cheaper, the RAC added.
It advised drivers to shop around for the best fuel deals rather than assume supermarkets are always the cheapest.
Diesel’s average price in September dropped from 3.5p to 180.2p.