Road wear is generally represented as the square of vehicle weight. A 6000 pound vehicle will do 4x as much wear on the road surface as a 3000 pound vehicle. All of this pales in comparison to when a 60,000 pound cement mixer rolls through, but then again, the overwhelming majority of vehicles on commuting roads are passenger vehicles between 2000 and 7000 pounds.

The bottom line is I think this is a bad idea. If they're hurting for tax revenue then RAISE THE GAS TAX!! Gasoline is getting far more expensive these days anyway so when gas is $2/gal instead of the $1/gal it was 5 years ago, raising it 10 cents is only half as much of a price increase in percentage as it would have been back then. Raising the price of fuel will motivate people to buy vehicles that get better gas mileage, and those vehicles tend to be lighter anyway.

Not to beat a dead horse here, but the other big advantage of the gas tax is that it's super low tech, meaning it costs virtually nothing to implement a tax increase. Compare that to the HUGE costs of setting up a super high tech GPS tracking system that's going to be prone to errors, server crashes, and hacking to avoid taxes. There's no way to cheat at paying the pump tax though; at least not that any significant number of people would ever attempt. Raising the gas tax is the way to go.