But the reduction, and a forecast 2p dip in diesel, will only happen if other retailers follow the supermarkets' lead in dropping prices, the RAC added.
It said that the wholesale price of petrol had now reached its lowest price since the beginning of December 2012.
On September 17, when the Syrian crisis subsided and the pound was at a seven-month high against the dollar, the average price of petrol was 137.64p and diesel 142.50p, the RAC said.
By October 6, petrol had dropped to 132.34p and diesel to 139.00p.
It said the two-week pricing forecast is for a reduction of almost 4p in petrol and nearly 2p in diesel which would mean average prices of around 129p and 137p respectively - taking prices below previous lows for the last year.
The RAC went on: "The supermarkets have, however, already brought their fuel down to this level, demonstrating transparency by passing on the wholesale savings to motorists, so the new predicted average figures will only be achieved if other fuel retailers continue to reduce their prices."
Petrol reached a 12-month average retail low on January 8 2013 at 131.93p whereas diesel's low came on May 13 at 138.05p. Since May, however, prices of both fuels began to slowly rise again until they were reversed by the mid-September reductions.
RAC head of external affairs Pete Williams said: "Fuel retailers have clearly demonstrated the transparency of their operations by cutting more off a litre of fuel than many people will be able to remember. As well as saving people hard-earned cash at the pumps, this level of transparency has no doubt created a lot of goodwill with their customers.
"Motorists often complain that prices seem to go up far faster than they come down, but this autumn is proof that this is not necessarily the case."