Petrol prices fell by an average of 9p per litre last month, according to the latest figures, with diesel dropping almost 7p.
A shift in prices after months of constant rises knocked almost £5 off the cost of a tank of unleaded and more than £3.50 off a full fill-up of diesel.
However, motoring groups have said the drop should have been sharper and quicker, and warned that drivers were still being overcharged.
After surging to record highs at the start of July, petrol and diesel prices fell back throughout the month as wholesale prices fell. Petrol dropped from a record high of 191.53p per litre at the start of July to 182.69p and diesel declined from 199.07p to 192.38p by the end of the month, according to analysts Experian Catalist.
The drops represent the third and fourth biggest monthly reductions in the last 20 years - 8.74p and 6.69p respectively - but the RAC says that with wholesale petrol prices now at the same level as early May, drivers are still being overcharged by 16p per litre.
It and fellow motoring group the AA estimate that a litre of unleaded should cost between 165p and 167p per litre, compared to the actual average of 182p.
The RAC says that even allowing a two-week period for the price change to reach the smallest retailers, the reduction has been far too slow.
Its fuel spokesman, Simon Williams, said: “July has been an unnecessarily tough month for drivers due to the big four supermarkets’ unwillingness to cut their prices to a more reasonable level, reflecting the consistent and significant reductions in the wholesale cost of petrol and diesel.
“What ought to have happened is that the biggest retailers cut their prices more significantly on a daily basis. Instead, average retailer margin for petrol across the industry has been up around 20p a litre for the last two weeks – more than three times its long-term average.”
Supermarkets have begun cutting their prices in recent days but the AA’s Luke Bosdet said only Asda had lowered prices to a fair level and there were still significant savings to be passed on to drivers.
Cheapest and most expensive regions for fuel
The latest figures show large geographical differences in both fuel prices and the extent that savings have been passed on.
Drivers in Northern Ireland enjoyed the sharpest drops and the lowest prices in July, with petrol falling 12.33p per litre to 178.36p and diesel dropping 11.61p to 186.07.
England’s North West saw the next-largest reductions with cuts of 9.77p and 7.35p respectively but those in London and England’s East and South East saw the smallest cuts, with petrol dropping between 7.6 and 8.3p and diesel falling just 5.5p.
Mr Williams advised drivers searching for the cheapest fuel near them not assume that supermarkets were always the best value.
He said: “The best advice for filling up is no longer to assume the supermarkets are the cheapest, but to shop around as it’s highly likely you’ll find an independent retailer which is doing the right thing and fairly reflecting their lower wholesale costs by charging a lower price.”