The average price of petrol fell by nearly 7p a litre to 162.89p in September but drivers would have seen a further 10ppl reduction if retailers hadn’t upped their margins.

A tank of unleaded has dropped below £90 for the first time since May – a diesel fill-up is now under £100

New data from the RAC shows a 55-litre petrol fill-up has now dropped below the £90 mark for the first time since the start of May. Compared to the peak high of 191.53p on 3 July, drivers are saving nearly £16 on a full tank.

A litre of diesel also reduced in September but only by 3.5p a litre, finishing the month at 180.16p. Diesel is now 19ppl cheaper than its all-time high price of 199.09p recorded on 25 June and a full tank of fuel is now under £100 (£99.09) – something drivers haven’t seen since late May.

The average unleaded price at the big four supermarkets also fell 6.6p a litre to 161.30p last month – but this is now only 1.5p cheaper than the UK average, compared to 3.5p normally. Supermarket diesel only fell 1.4p to 178.56p, which is now only 2p under the UK average – diesel is also normally 3.5p cheaper at the big supermarkets.

Despite September seeing the sixth biggest ever drop in the price of petrol since 2000, the RAC says the fall should have been much bigger. Retailer margins on unleaded are a huge 10p more than normal – 17p a litre compared to long-term margin of 7p.

Fuel spokesman Simon Williams said the data points heavily to retailers not playing fair with drivers.

“This is backed up by the fact that Morrisons, which was charging an average of 162p a litre for petrol at the end of September, is currently running a 5p a litre off promotion for customers spending £40 in store. This type of promotion tends only to be seen when supermarkets are benefiting from lower wholesale prices.”

And with many smaller forecourts now selling fuel much cheaper than the supermarkets, the RAC continues to urge drivers to shop around for the best deals rather than simply assuming the supermarkets are the lowest because they have been in the past.

It also warned that the current weakness of the pound means fuel prices are likely to remain volatile – if the price of oil starts rising, both petrol and diesel prices could rapidly start going back up again.

“With oil producer group OPEC and its allies meeting today much will depend on how big their expected production cut turns out to be. But it goes without saying that any further output restriction will not be good for drivers as it will inevitably – as intended – push up the barrel price, ultimately making fuel more expensive,” added the RAC.

The Petrol Retailers Association responded to the RAC data, highlighting that fuel retailers are dealing with increased operating costs driven by a 40-year high inflation rate and rocketing energy prices, hence the increased fuel margins.

Gordon Balmer, executive director, continued: “I was pleased to see that the RAC have highlighted that independents and smaller forecourts are more competitive than the supermarkets. My advice to motorists is to shop around.”