Fuel retailers have been urged to "immediately" ease pressure at the pumps by passing on a drop in wholesale prices to motorists.
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.
"While supermarket promotions based on money spent in-store in return for a 5p a litre discount coupon at the pumps are beneficial, these are very often only used by retailers at times when the wholesale price of fuel has eased.
"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.
AA president Edmund King said the key issue affecting motorists at the forecourt was the disparity in the cost of petrol between neighbouring towns, in what he described as a "pump price postcode lottery".
"Last weekend, supermarket petrol in some towns was 5p a litre more expensive than supermarket petrol in another town just down the road," Mr King said.
"Why many drivers have to pay at least £2.50 a tank more for the cheapest fuel in their area is beyond the understanding of everyone except supermarkets and the fuel industry.
"The falling price of oil and stronger pound is cutting the cost of fuel but the new reality is that market volatility could easily push it the other way.
"The pump price postcode lottery is a persistent menace and the AA is now providing millions of its members with a smartphone tool to compare prices between towns and vote for fair fuel prices with their wheels."