The RAC has warned that people who fill their cars up with diesel could be paying far more at the pumps than they should.
Diesel is 17p more expensive than petrol at forecourts despite the wholesale price being the same.
According to RAC Fuel Watch, the average price of a litre of petrol is 146.63p while diesel is priced at 164.26p typically.
However, both fuels are selling for around 114.5p on the wholesale market, the RAC said.
At the beginning of March this year, the RAC found that diesel was only 6p more expensive than petrol but there was a 20p a litre gap between the fuels at the forecourts.
The motoring group said the overcharging of diesel vehicle owners by retailers is 'absolutely shocking' and blamed the big four supermarkets for inflating forecourts prices.
Given the amount wholesale prices had dropped, forecourts should have already reduced pump costs for diesel to about 152p the RAC said.
A further cut to 147p should then be seen in the coming weeks.
Simon Williams, fuel spokesman at the RAC said: "Sadly, this seems unlikely given current retailer behaviour. Instead, the big four supermarkets, which dominate UK fuel retailing, are charging an outrageous of 162p a litre on average."
When prices change in the wholesale market, it can take some time to feed through to changes at the pumps, due to how frequently smaller sellers restock.
However, Simon says this should not be the case for the larger supermarkets.
He added: “As the supermarkets buy so frequently they have had plenty of time to pass on the lower prices they are benefitting from on the wholesale market to drivers at the pumps, but they remain totally resolute in their refusal to cut their prices substantially which is nothing short of scandalous, particularly in a cost-of-living crisis."
"We are seeing many independent retailers charging far less than their supermarket rivals which is a sign of how much fuel retailing has changed.
"This would have been pretty unusual several years ago but is now rapidly becoming the norm.”
The RAC says the retailer passing on the most savings of falling wholesale diesel costs is Costco.
It is currently charging less than 150p per litre for diesel - though you have to hold a Costco membership in order to fill up at one of its fuel forecourts.
