It is a car tax.

'Duty' is simply a form of taxation that is levied on objects rather than individuals - Vehicle Excise Duty can be correctly described as a car tax.

Some confusion arises because vehicle excise duty was held in a 'separate account', (hypothecation), used to fund road building and maintenace - it came to be commonly known as 'road tax' despite it actually being a tax on vehicles.

Strange that many still refer to 'road tax' as it has been the best part of 80 years since it could be described as such - hypothecation ended in the 1930s

As the tax is levied on vehicles, not how much they are used, there is no real argument to suggest this tax should be transferred to petrol or diesel or how much they are used.