The large vehicles are already paying a 'weight-mile tax' instead of the flat fee you pay at the pumps.  

What is different about your tax for them? 

 "MILEAGE TAX RATES "  ODOT.

http://www.oregon.gov/ODOT/Forms/Motcarr/9928.pdf

Where are you going to get the actual money to 'subsidize' LTD even further?  

They now have an 'employer' tax, and effective this year they have a new 'employee' tax and then there is the  Federal tax money they get.