Researchers have found that as few as one in 10 petrol stations are charging a fair price for fuel.
A recent drop in oil costs means that prices at the pumps should be below the current average of £1.88 per litre by more than 10p.
RAC analysis found that unleaded petrol should currently be retailing in the region of £1.74 per litre.
Experts also found that independent local forecourts were currently giving drivers a better deal than the big petrol companies, The Sun reports.
The RAC looked at 4,500 forecourts and found that only 157 of them were selling petrol for between £1.70 and £1.79 per litre – with independent forecourts making up 125 of that total.
Diesel drivers are also being overcharged according to the research, which says the current price should be around £1.89 a litre rather than the average of £1.96.
There were only 250 providers selling diesel at a fair price, of which 197 were independent.
RAC spokesman Simon Williams said in quotes reported by The Sun: “Weekly wholesale petrol prices have fallen by a massive 17p-a-litre to 135p-a-litre.
“Yet average pump prices have reduced by a paltry 4p.
“It’s time for every retailer to do the right thing and cut their prices to more reasonable levels.”
The findings come as fuel price protests are set to make traffic jams even worse during what is expected to be the busiest summer getaway in at least eight years.
Avon and Somerset Police warned motorists that “slow-moving roadblocks” are planned on parts of the M4, M5, M32 and A38 on Friday morning.
An image posted on Facebook group Fuel Price Stand Against Tax suggests demonstrations will be held “nationwide”, including in Birmingham, Cardiff, Liverpool, London and Manchester.
With most schools in England and Wales breaking up for summer this week, the RAC said an estimated 18.8 million leisure trips are planned in the UK between Friday and Monday.
That is the most since the company began tracking summer getaway numbers in 2014.
Fuel price protests on July 4 led to 12 people being arrested on the M4.