"Road Fund Tax" has been a misnomer for years now. The licence duty is a form of tax going into the general government pot and is no longer used to improve the appalling state of our roads.
The VED bands similarly bear no relationship to the "green"ness of the vehicle - just look at the DVLA lists and you'll see smaller engined cars paying more than larger simply because they are allegedly creating more "emissions" .
It's all a con under the cover-all of eco-friendliness.;