Was talking about this with someone the other day, the only real fair way of going about it is to get rid of the cost of your car tax and add 25p tax to the price of fuel which in turn gets used for what your road tax would of been used for.

I have 4 cars which I keep taxed, 2 of them get used everyday by me and the otherhalf and 2 which maybe get 1-2k miles a year, yet I still pay over £400 to keep them taxed.