Filling up at a motorway service station can cost the average motorist an extra £8 a tank compared with elsewhere, the RAC has said.
The motoring organisation says motorway prices can be 10p a litre above the national average.
It wants retailers to cut their prices, or be forced to do so by the government.
The Petrol Retailers Association said the prices were justified, because of the high costs of running the sites.
The RAC's Simon Williams said the motoring body could see "no reason why motorway fuel should be so much more expensive".
"In fact arguably it is much easier from a delivery point of view than it is getting fuel to urban delivery stations," he said.
In extreme cases, the RAC said petrol and diesel could be 15p a litre more expensive on the motorway.
The UK has four big motorway service station operators, Welcome Break, Moto, Roadchef and Extra. Three of them are owned by venture capitalists.
"These service areas cost a fortune to build," said Brian Madderson of the Petrol Retailers Association.
"One, on the M5 in Gloucestershire, cost £30m," he added.
As far as profits are concerned, he said margins on diesel for HGVs and buses were very small, although they were likely to be higher on fuel sold to motorists.
"The operators will get a reasonable margin - but only to balance out the costs of operating a motoring forecourt," he said.
According to the RAC, many motorists only fill up at a motorway service station when there is no other choice, or else they put in just enough "to get by".
That increases the risk of drivers running out of fuel.
The organisation would like prices to be advertised in advance on motorways, as they currently are in France, and once were in the UK.
In May 2013, David Cameron asked the Number 10 policy unit to investigate making this compulsory once again.
But the RAC said there had been no progress on the issue, and no update on tackling high fuel prices.
The Department for Transport said it was planning a trial of motorway signs next year.
It said the signs, which will display the cost of petrol and diesel, would boost competition, and therefore reduce prices.
It has not yet decided where the trial will take place.
Last year, the Office of Fair Trading (OFT) reported that petrol prices on motorways were 7.5p a litre higher than elsewhere, and diesel prices were 8p a litre more expensive.
However, the OFT concluded there was no evidence of price fixing.