Of course the more fuel efficient the car the more marginal the benefit. Online calculators suggest the tax paid on the free fuel benefit (not the company car benefit) on a Touran would be around £665. Enough money, at the Touran’s mpg, to fund 7,341 miles of private use. So if you do 150 miles a week you’re getting a financial benefit equivalent to a tank of diesel over a year. There is the convenience benefit of cashless fuel that is probably worth some psychologically.

The benefit from having the company car itself is more tangible in that it avoids the initial capital outlay on a second car. (via coComments by mazportico)