I do about 17k.
I paid £5k for my 406 5 years ago. I've covered 85k miles and spent £7k on 6,672 litres of diesel.
Thats 1,467 galons.
The XUD 406 I had previously averaged 46mpg, so it would have only done 67.5k on that fuel.
To do 85k would take it 1847 galons, which would cost say £7k * 1,847 / 1,467 = £8.8k.
Call that a £2k saving, then I have to do a further 130k miles before I can justify the swap soley on the gounds of efficiency. At the current rate, that will take about 7 years and take me to 300k on the clock

So my new year's resolution will be to drive more
