The RAC is demanding action to cap motorway service station fuel prices, describing them as "21st century highway robbery."
The motoring group said its research found that petrol and diesel costs can be over 15p-per-litre more expensive and it was unacceptable that drivers had to pay such a premium.
Motorists told the RAC's survey they felt they were being "held to ransom" with one-in-four saying they refused to buy fuel at services and almost one-in-two admitting they only used motorway stations as a last resort.
The RAC said almost two-thirds of those questioned wanted Government or industry action to cap fuel, so it was no more expensive than on non-motorway forecourts.
The organisation said the results highlighted "real concern" about drivers risking their fuel tanks running dry rather than filling up at motorway services, posing a danger to themselves and other motorists.
Its fuel spokesman Simon Williams said: "It's no wonder that motorists feel held to ransom with prices on the motorways inflated to such an extent.
"In some cases motorway petrol and diesel might even be 15p dearer than the cheapest forecourts, which would add as much as £8 to the price of a tank of an average family-sized vehicle.
"We can 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 filling stations.
"We're calling for motorway fuel retailers to be more reasonable with their pricing."
It also suggested service stations were forced to display the price of their fuel well before the exit so drivers knew how much it would cost to fill up before leaving the motorway.