So for 1 year that's 1,514L. And 1L of super unleaded = £1.43, so you're spending £2,165.02 per year on petrol. You can make your own figures up for insurance and what have you - let's call it £5,000 per year, averaged out. Assuming the Tesla is £80,000, you don't break even until year 6, assuming the Tesla's running costs are £0/year.

If you do drive a lot you'd probably buy some kind of diesel car, so then you could expect more like 30mpg, perhaps, assuming you're going for the equivalent of this 20mpg thing. And the break-even part recedes further into the future, or you can just drive more.

(I've no real feel for what sort of car you'd get for £50,000 though. At a guess: BMW 530d, Jaguar XF, Range Rover TDV6...? Something like that. Are those really the Tesla Model S's natural competitors?)