Sure, weight x distance is generally fairer than just direct gas usage. But if we're going to go there, why not do it properly? Damage to infrastructure is proportional to the 4th power of weight; thus, we should probably tax something like ([miles travelled]/1000miles)*([vehicle weight]/1500lbs)^4 for vehicle registration. That would take into account the proper damage. The average american drives 13476 miles and the average fleet curb weight (in 2004, latest i could quickly find) was 3239 lbs; this would give a result of $293 for registration. If you drove the same amount in a vehicle half that, you'd pay like $17, and if you drove a vehicle twice that weight you'd pay $4466. That would take into account proper damage incurred on infrastructure.
From the article: "Our research found that the fourth power is often inaccurate, " continuing: "More importantly, pavement serviceability is often not the most relevant measure used by highway agencies to trigger maintenance activities."
You knew the job was dangerous when you took it, Fred. -- Superchicken