A South Australian peak motor industry body supports a move to "name and shame" petrol retailers in the country.
The Royal Automobile Association (RAA) wants falling wholesale fuel prices to be passed to motorists.
This comes as a gap between wholesale fuel costs and pump prices doubles in rural areas.
The body backs national consumer watchdog the Australian Competition and Consumer Commission which warned it would highlight individual retailers not passing on falling wholesale costs.
According to RAA, this month wholesale unleaded petrol prices have fallen by 36.9 cents per litre - from 125.9c/l to 89c/l.
But average pump prices in regional SA have dropped by only 16.7c/l - from 143.9c/l to 127.2c/l, RAA reveal.
This has seen the gap between the average pump price and wholesale price in regional SA jump from 18c/l to 38.2c/l.
RAA fuel expert Mark Borlace said wholesale price savings must be passed on to country motorists as the economic impact of the Covid-19 pandemic worsens.
"Wholesale price cuts do take longer to pass onto country bowsers due to lower sales volumes compared to Adelaide, but the rate of price reduction is unwarranted,'' he said.
He used Whyalla as an example, where prices this month have dropped about 10c/l on average, despite wholesale costs being slashed by more than 38c/l in the same time.
RAA figures further show petrol pump prices falls have varied across the state.
In contrast to Whyalla, Port Lincoln's average prices have fallen by almost 23c/l - from 148.5c/l to 125.6c/l - this month.
Mr Borlace said diesel pump prices had also failed to fall as far as they should, both in Adelaide and the country.
Wholesale diesel prices have dropped about 19c/l while the average pump price in Adelaide and the regions has fallen by around 12c/l
"We have raised our concerns with the national watchdog about the slow rate at which fuel cost savings are being passed on to motorists,'' Mr Borlace said.
"This is why we are encouraging retailers to pass on savings as soon as possible.''
Fuel retailer giant On The Run was approached for comment by Australian Community Media, however are yet to respond.