VED hasn't been specifically for road maintenance since it's stopped being called Road Tax. It's a 'Green' tax based on vehicle ownership that all goes into a big pot along with fuel duty and probably most other taxes, to be distributed however the treasury sees fit.
It wasn't so long ago when 'Road Tax' bands where based on the type of vehicle - motorcycles, passenger vehicles, commercial vehicles etc (reliant robins were actually classed as motorcycles and my dad did try to convince me to buy one

). Then it became linked to engine capacity, and now it's based on CO2 emissions.
Come the time when most cars are below the 120g/km threshold, the goalposts will either move down, or it'll be based on other emissions as well as CO2.
If you used V power or BP Ultimate, which increase fuel economy, you'll pay less tax through fuel duty because you won't be filling up so often, though with the prices of these premium fuels, I'm not sure how worthwhile it is.
BTW, you'll be paying 15 times more VED than me next year.
