What, they don't use more gas / pay more gas tax than the rest of us?
Not in proportion to the wear on the roadway they produce. I think the roadway wear goes as either the square or the cube of the weight per axle, and the big trucks weigh a lot more per axle. Nope - looks like it is a fourth power relationship:
"Road damage rises steeply with axle weight, and is estimated "as a rule of thumb... for reasonably strong pavement surfaces" to be proportional to the fourth power of the axle weight."
It looks like the max axle weight is something like 20,000 lbs. while the average passanger vehicle weighs something like 3500lbs, which would be under 2000 lbs per axle. Thus each very heavy truck can be even more than ten times the axle weight of the average car - and 10^4 is 10,000, so that truck can cause as much wear as 10,000 cars. Or maybe that 10,000 factor is per axle (five axles in an "18 wheeler"), so maybe it is a factor of 25,000 when comparing an 18-wheeler to a car.
Wow - trucks really tear up the roads!