This should be a per-mile fee that varies based on weight, modeling road damage. Set the tax to specifically counteract the implicit subsidy.

Otherwise you are taxing diesel sedans and tractor trailers the same amount per gallon when a gallon to the latter does far more road damage.

Actually fuel it’s a good proxy for that: its use is proportional to mileage and weight

	▲	jefftk an hour ago \| parent [-]
		But fuel usage it not proportional to mileage and weight across vehicle classes, and damage is proportional to axle weight to the fourth power. A diesel sedan might use 1.25 gal for 100 ton-miles, while a tractor trailer might use a third of that (though, to be fair, I thought this would be a bigger delta before I looked). But then if you look at "Equivalent Single Axle Loads", a tractor trailer is ~2 ESAL while a diesel sedan is ~0.0003 ESAL. So we're talking maybe 0.13 ESAL/gal for the former vs 0.0001 ESAL/gal for the latter.