Quite a few years ago I did a bit of fiddling with a calculator and came up with putting an extra 9p on fuel, working on an average of 10,000 miles a year for the average car at around 30mpg. The 9p would have covered the road tax at the time, and would have removed a potential crime from the books. Unevadable, too, unlike the tax discs, and would have benefitted the archetypal little old lady types who only drive to the shops once a week while catching those who do massive mileage and contribute more than most to wearing out the roads in the first place. Still, it's better to have a nice big bit of pointless bureaucracy around, I suppose.