RESEARCH by NRMA Motoring & Services has found that a widening gap between the price of regular and premium unleaded petrol has resulted in motorists being over-charged by up to three to four cents per litre on premium fuel.

The NRMA research analysed average weekly price movements for regular unleaded in Sydney against premium unleaded petrol (95/96 and 98 Octane) from 28 June 2006 to 13 September 2009.

When the gap between regular and high grade fuels hit its peak in August 2009, motorists were being charged almost eight cents per litre more for 95/96 Octane and almost 12 cents more for 98 Octane.

The NRMA believes the gap between the various grades of fuel should be at least three to four cents less per litre than is currently being charged.

NRMA Motoring & Services President Wendy Machin said the widening gap between the costs of fuels was disturbing and worthy of an investigation by the ACCC.

"On 29 July 2007 the weekly average gap between regular and 98 Octane petrol was around nine cents per litre. On August 30 2009 the gap was almost 15 cents," Ms Machin said.

"Clearly something is not right. We have taken the extra cost associated with refining high grade fuels into consideration and there is still a considerable gap.

"Many motorists have no choice but to use high grade fuels for their vehicles and it appears they are being over-charged unnecessarily. That's why the NRMA has written to the Petrol Commissioner asking for an investigation into the price discrepancies."

The gap between the two premium grades against regular unleaded continues to widen.

Ms Machin said unless action is taken motorists who use high grade fuel would be forced to pay higher prices in the future.

"NRMA is concerned that this gap between different fuels will continue to widen, especially after E10 fuel becomes mandated from July 2011 and regular unleaded petrol is phased out of the market," Ms Machin said.

"This means motorists will be left with even fewer choices at the pump and the NRMA fears many motorists will be required to pay relatively more per litre for premium unleaded petrol."