The problem with any tax system is that it can be simple or it can be fair, but it can't be both.

To be simple every road user pays a similar amount of road tax. To be fair everybody pays by the mile travelled. Personally, I'd prefer the fair system, because I'd make sure that I would not get excessivly hit by the higher rates.

There are good reasons why, as you say, that, for example, people in rural areas may need to drive large (and less efficient) vehicles. This is why I favour the fair system. These people shouldn't be peanalised by being, for another example, a thatcher who needs to transport tools and ladders and staw around rural areas: things like those don't easily go in a small van and an SUV is probably the best vehicle for the job. People who run stables tend to have a genuine reason for using Range Rovers and the like.