If the tax were abolished and the government raised the fuel duty to compensate, the most tax would automatically be paid by those with the biggest cars, those doing the greatest mileage, and those on the most congested roads. (If you look in your vehicle handbook it will probably tell you that if you're in second gear your mileage per gallon will be about half what it would be in fourth gear. In first it will be more like one third.) If your gearbox is automatic the same applies; if you go too slowly you waste fuel automatically.
If the powers that be really wanted to reduce carbon dioxide emissions, they wouldn't keep putting mini-roundabouts where they're not needed, reducing the number of lanes, closing roads so that traffic is concentrated and congestion is created, and introducing unnecessary one-way systems to make motorists drive round two sides of a triangle or three sides of a rectangle for no good reason. They wouldn't put 24-hour traffic lights on roundabouts. You'd be able to drive over speed humps at 25 m.p.h. in a 30 limit without damaging your vehicle and not have to waste energy braking to walking pace and crawling about in first gear. You wouldn't fail your driving test for coasting.