> Abolish road tax and put it on fuel instead.
Not the smartest idea. It is both unsustainable and regressive. It is unsustainable because a significant percentage of vehicles in the next decade will be PHEVs or EVs. We can agree to disagree on the rate of growth of these categories, but price is coming down, choices are increasing, range is increasing and a shed load of money is going into R&D, so it will increase over time. That also drives the regressiveness. It is the wealthy who can afford such cars, so they are the ones getting the tax break. The poorer folk fighting to keep some old rust bucket alive are the ones who get hit with this tax worst (that is true even today) but because of the reduction in tax take, the rate gets increased to retain the total revenue.
We saw something similar here (down under) with our power grid and the growth of home air conditioning. Back 30+ years, it was somewhere near 1 in 4 houses that had it. Today, every new house/unit has it almost without exception. Every renovation adds it. The extra draw on the grid means that* the distribution gets expensive upgrades to cope with the <50 hours a year where all those units are simultaneously on. Furthermore, old, unreliable, and inefficient powerplants at retirement age get billions pumped into them to keep them on life support for another 5 years. It's hard to complain** if you are sitting their in your A/C, being part of the primary cause of the demand side of things that requires. It is quite another (perverse) thing when you are not wealthy enough for A/C yet your pet bill has doubled in the last decade (and then some). The wealthy will respond to this by buying solar panels and battery storage, meaning all that investment capital gets recouped by those who can't afford to go solar/battery. It's a death spiral. So back on point, by all means have a sales tax on inefficient vehicles in the first place, or incentives to buyback old inefficient vehicles, or per Km billing, but don't put it on the fuel itself.
* not the only reason, also some gold plating going on.
** actually, it doesn't seem that hard at all