It needs to be a forumula that is based on miles driven AND weight of the car... unless Oregon actually believes that a Civic and a Big Rig cause the same amount of long term damage to a road.
But they they would either have to charge a semi a lot, or not collect anything from the Civic, in order for it to actually be based on how much road damage the vehicle causes. According to this GAO study: http://archive.gao.gov/f0302/1... a semi causes 9,600 times the road damage of a car. The article says the tax is 1.5 cents per mile. That means if the Civic is getting charged 1.5 cents per mile, then a semi should be charged $144 per mile. Or, if the semi were charged 1.5 cents per mile, then the Civic would be charged .00015625 cents per mile, and thus would have to be driven 3,200 miles in order to get to half a cent which could then be rounded up and charged a full cent.
In reality, this "fee" is just a workaround for people whining about raising the gas tax. Just raise the fucking gas tax. If it makes you feel better, call it the "fuel usage fe.e" Either way it's largely a subsidy for truck transportation.