Per Gallon Gas Taxes

Gas taxes are typically calculated on a per gallon basis. This makes them different from the vast majority of sales taxes, which typically are calculated as a percentage of the purchase price. I can see plusses and minuses to calculating sales taxes on a per item basis rather than a percentage basis.
The plusses are relatively simple. Pigouvian taxes are supposed to internalize externalities. Petrol has some obvious externalities-- global warming, costs of securing oil supply, etc. These costs are not obviously related to the cost of oil, so it makes sense to charge a fixed price tax regardless of the cost of oil. Moreover, externalities are different for every product, so why charge the same percentage regardless of the item.

There are also some minuses with fixed taxes . One is that its very costly to specify a tax for every product, so some proxy, like purchase price, might save a lot of effort and also avoid having to draw a line between what is product A versus product B. (I don't think these are big problems for gas, but I could be wrong.) Another problem, quite relevant for gas, is that fixed rate prices might need to be adjusted more frequently than other prices and this might be politically difficult. Gas taxes haven't changed much in recent years, in spite of the fact that oil is way more expensive and the perceived externalities of oil in gl9bal warming and national security terms appear to have grown.

The end result is that today gas isn't really taxed much more heavily than any other product in spite of its unique externalities, while in the past gas has been heavily taxed in relative terms.  The average total gas tax  (state and federal) is around 42 cents. With average retail gas prices at about $3.22, the effective tax rate is around 15%. In 2002, average retail gasoline prices  were around $1.43, so the effective tax rate was around 42%. This is an enormous percentage change, and I don't think its been made through any deliberate action.

Thoughts always welcome.