The Automobile Association says drivers are paying too much for petrol because of a law change last year.
In the past, part of the petrol levy was diverted into the general Crown account.
Since the law change, the money goes directly to the National Land Transport Fund, which pays for a variety of roading related projects including policing and state highways.
AA spokesperson Mike Noon says as a result, petrol motorists are paying a greater proportion than previously towards transport costs through the levy and other charges, while those who use diesel are unaffected.
The contribution to the cost of the roading network by motorists driving petrol-fuelled vehicles has risen from 55% to 63%, he says, while truckies pay 37%, down from 45%.
Mr Noon estimates petrol motorists are proportionally paying about eight cents a litre more than they should, or almost $300 million a year.
He says to remedy the situation, they either should pay less, which is unrealistic, or diesel drivers should pay more to meet their share of the costs.
Road Transport Forum spokesman Tony Friedlander says if anything the law change brought truckies into line with their counterparts in other countries.
The issue of apportioning costs was considered by the Road User Charges Review Group, whose report to the Government has yet to be made public.