Currently about half of the tax is assessed as a per-gallon charge that changes according to the price of gas. That means it's highly volatile: if the price of gas skyrockets, so does the tax. The state Revenue Department sets the tax about twice a year, The Atlanta Journal-Constitution reported.

The other half of the tax is also a per-gallon charge, but it never changes. Set in 1971 at 7.5 cents per gallon, it is still that number today.

Under the new proposals, both parts of the gas tax would be pegged to the cost of road construction. So drivers would pay a per-gallon charge based on a once-a-year reckoning by the state of the cost of road-building.

The last decade included ups and downs both in the price of gas and in the price of highway construction. During the mid-2000s building boom, the price of construction materials increased. As oil prices rose, the price of asphalt tar soared.

The gas-tax revision is part of a larger proposal to change the state's tax structure. The Special Council on Tax Reform and Fairness for Georgians studied Georgia's taxes for months before making its proposal to the Legislature. The bill filed last week, HB 385, embodies all the proposals the council offered, although it gives the revenue commissioner some leeway in setting the tax rate.

According to council member David Sjoquist, if the new proposals had been in place since 2000, the state gas tax would now be about 6 cents per gallon higher, a 40% increase. That means a 15-gallon tank of gas would cost 90 cents more than it does now.

For state road projects, a 40% increase in fuel-tax collections would add up to about $300 million extra a year.