Look at a simple solution and don't complicate it with baggage. You cannot regulate the buyers behavior. For a new vehicle, charge $1 use tax for every pound over 2800 and credit $1 for every pound under 2800. For annual registration, charge or credit $0.10 per pound. Apply this to every vehicle without exception up to 10,000 pounds. For every pound over 10,000 reduce the fee by 60%. Use the revenue to fund private and public research. Allow organizations to request grants from this fund to support their research. People will ask for lighter vehicles.