Low tech solution?
http://www.cyberguys.com/templates/searchdetail.as p?T1=121+2570&dept=tch2&search=1ca54&child=

So both my Ford Focus and Giant Urban Assault Vehicle get to pay the same tax, sure the UAV uses more gas, and tears up the road more, but if we both put 50 miles on the cars, then we both get taxed the same.