Some drivers have been getting a “lucky dip” when filling up with petrol, with some pumps supplying more fuel than the counter indicates, the AA has said.
Some fortunate motorists have been getting as much as 4.4% more petrol than shown on the pump, the AA revealed.
This is effectively a 6p-a-litre gift to drivers at a time when fuel prices are extremely high.
The AA said the discrepancies had come to light after AA members feared they were not getting enough petrol for their money.
It added that trading standards officers had reported discrepancies at four of 38 sites in Cumbria and 11 of 216 nozzles in Derby.
In Newport in South Wales, just two of 353 nozzles failed - one of them giving more fuel than indicated.
The AA said an EU ruling that will eventually force retailers to upgrade forecourt equipment to meet the new Measuring Instruments Directive, coming into force by the end of October 2016, had created an incompatibility problem for forecourts with a mix of ageing and newer pumps.
AA president Edmund King said: "Following concern from members, the AA carried out preliminary tests at a handful of fuel stations to see if there were grounds for a more comprehensive survey.
"Not only did all the pumps dispense at least the right amount, but most gave even more."
He added: "Some drivers have been benefiting from a lucky dip at the pumps, getting more fuel than they pay for.
"However, crippling pump prices have produced desperation among poorer drivers, particularly those trying to get by on £10, £20 or £30 weekly fuel spends that their family budgets can barely afford.
"Petrol this week averaged 134.17p a litre - two years ago, it was 116.60p.
"Inevitably, some motorists are watching their gauges like hawks and complaining to Trading Standards."