The RAC said a litre of unleaded petrol sold wholesale for 6p less since the end of August, while diesel was down by 2p a litre, giving petrol stations the power to lower prices on the forecourt.
The car services company explained lower prices were due to a drop in oil prices since Syria agreed to sign an international treaty banning chemical weapons and the pound reaching a seven-month high against the dollar.
It comes as official figures showed inflation was down to 2.7%, partly due to a slow down in petrol pump rises.
Petrol prices rose 2p per litre compared with a rise of 3.5p per litre in the same month last year, according to the Consumer Price Index (CPI).
But Pete Williams, RAC head of external affairs, said motorists were watching to see whether retailers would reduce prices by sharing the benefits of lower wholesale costs.
He said: "If fuel retailers want the motoring public to fully trust they are operating fairly and transparently they should reflect the drop in wholesale prices immediately by cutting pump prices by up to 5p a litre for unleaded and 2p a litre for diesel.
"It would be far better for all fuel retailers to cut the prices so everyone can benefit. We realise this will mean less margin for the retailer, but the goodwill it creates through transparency would be beneficial in the longer term.
"Motorists are very aware that prices seem to go up far faster than they come down so this really is the time for the fuel retail industry to demonstrate that that's not the case."
Despite slower rises in petrol prices, Mr Williams said there had "never been a more expensive time to be a motorist" and pledged that the RAC would continue to lobby the Government to drive down costs at the pumps.
In a yearly report on drivers for the RAC, more than half (54%) said they would be able to visit friends and family more often and have a fuller social life if fuel was more affordable.