[Rich9600]

I've switched back to petrol at 10K pa with a 12 mile commute.

At the point of change I'd guess I was paying an extra £20-30 a month switching from a F30 330d to a F31 340i. Commuting 40mpg to 31mpg. Longer trip, regular 120 miles to in-laws dropped from 45mpg to 37mpg.

I had tanks mpg overlap between the two...

My 330d dropped fro 45mpg average at 15K pa to 40mpg average at 10K pa. I was expecting 28mpg from the 340i. However, the petrols 5 mile shorter warm up time makes it more economical than expected on my commute.

Both the 330d and the 340i get about 20mpg on a 2 mile run to the shops...

So, at 6K pa I'd suggest you'd be looking at £10-15 a month difference in fuel... £480-720 over 4 years...

With dieselgate ongoing I'd be much more worried about residuals and the GFV. Get your finance quotes before you rule petrol out. This will effect your monthlies more...