Take into equasion that no car out there has 100% difference in price for diesel vs petrol within the same model, so those calculations are slightly flawed.
It doesn't change the fact that with current fuel prices and 12,000 miles the difference between 35 mpg and realistic 50 mpg is £32-35 a month or ~385-400 a year. Find a petrol car that's £1000 cheaper than a diesel and you're ahead of the game.
Just keep an eye on two things:
- It's not true value of the car lowers insurance quote. In my case for example a £4,500 worth of 3 litre Toyota Hilux is half cheaper to insure than 1.6 Astra Estate. In similar fashion - very often stolen Pug 306 will be cheaper to insure than unwanted, thief deterring Citroen Xantia. There is no rhyme or reason to insurance grouping and prices and don't look for one, you won't find it.
- It's not true that insuring of petrol cars is always cheaper than of diesels.
So before you buy, check all the costs twice.