Taxing by Miles Driven is absolute nonsense when discussing 'road wear'.
The wear on the road rises exponentially as the weight of the vehicle increases. This must be included or all the light cars are subsidizing the wear caused by the heavy ones.
So accounting for the weight of the vehicle, we need to come up with units of (weight * distance)
For Example,
A Harley that drives a thousand miles would get something like ( 1000 lbs * 1000 miles ) = 1,000,000 lb*miles
And, a Semi Truck that drives a thousand miles would get something like ( 50,000 lbs * 1000 miles ) = 50,000,000 lb*miles
The tax rate would increase with the weight of the vehicle, accounting for the fact that the wear is exponentially growing with weight...
Then you take your 'lb*miles' value and multiply by the tax rate.
This is at least not completely moronic... but probably still doesn't reflect the true reality of the situation.