Some interesting maths I did about road use and road tax just now that I thought I would share. You are welcome to question my math and my methods.

I work under the assumption that the things that hurts roads is ground pressure. the force per square inch. more weight over less area. The same reason my feet hurt more as I get older. It might not be a solid assumption but I’m going with it.

Under that assumption we can assume that cars that have higher ground pressures do more damage to our infrastructure than cars with lower.

I tried to figure out how much my car, a Prius and a Full loaded Semi truck damage the road, and then with road tax, pay into the fund to fix it.

First up let me say figuring out ground pressure isn’t easy...or at least there were no easy calculators on the googles. I did find data on Semi trucks, but nothing on cars specifically, so for those I’m going with recommended tire pressure. Since air pressure carries the load, and its already in PSI I figured that would take the guess work out of it and make it even.

For my truck the recommended pressures are 32/32 (front/rear) so I assume a 32 PSI average ground pressure.

For the Prius its 33/35 (though forums tend to report that number as being low). So for the Prius we use 34 psi average ground pressure. Why is a 3100 lb hybrid less than my Cruiser? ‘Cause tire size. Less weight, but proportionally less contact patch = higher ground pressure.

Semi trucks seem to have 115 square inches per each dual tire, and 90 for each single. For an 18 wheeled tandem axle rig thats 1100 square inches. Given that max GCWR for many states is 80,000 lbs, Im using that at my number. Which gives us 72 PSI average ground pressure.

So a fully loaded semi presses down about 2x as hard as a Prius on the road. BUT, what does he pay into to the funds to fix and maintain those roads?

For my cruiser its between \$200-250 (my state gas tax is \$.245 nationally its about \$.184) [15,000 miles/yr @ 13 mpg]

For the Prius its between \$117-147 [fueleconomy.gov estimates 600\$ of fuel/yr]

For the fully loaded Semi its about \$1730 [45,000 miles/yr @ 6.5 mpg \$.245 tax rate]

Boil it down per mile

Cruiser - \$.015/Mile

Prius - \$.0088/Mile

Semi - \$.038/Mile

Using the ground pressure of 30 PSI = 1 as a damage metric [I just made it up but it seemed to hit a generic “car” level] means:

Cruiser - 1.06X damage

Prius - 1.13X damage

Semi - 2.4X damage

Now I realize this probably doesn’t scale linearly, but its easier this way.

Factor in who pays into the system though and it works out like this.

Cruiser - \$.014/Mile damage factor adjusted

Prius - \$.0033/Mile damage factor adjusted

Semi - \$.016/Mile Damage factor adjusted.

So yeah, Semi’s dent the hell out of the road, but they also pay more pack into the fund to fix it than most people realize. And Prius driver? You’re doing diddle squat so you can take your big truck hate elsewhere.