I want to see motor vehicles taxed by the volume they occupy.
Multiply the length x breadth x height together, divide by number of seats, charge it annually. (Allow a discount for commercial vehicles that carry freight, but NOT for pickup trucks or vans used for general business, i.e. the dentist driving a tricked-out Ford F150 for the tax break.)
Such a tax scales as the cube of the vehicle's overall dimensions, so it punishes oversize SUVs and pickups accordingly.