I actually keep two spreadsheets for both cars, and put in all monies spent on everything, from fuel to purchase price to service,tax, insurance etc... I also put in the miles every time i spend, and for the two cars i then can work out the MPG each time i fill up (can be useful for diagnosing probs too like when the brakes were binding, the mpg went down by 10mpg so i looked into it!) .
For the Golf i have spent 33p every mile i have covered, and since 2005 when i started i have averaged 32mpg, in 2009 average price per mile was 12p, now it is 19p.
For the Fox, i have spent £1.80 every mile, but that does include the cost of the csr too, and it is coming down very quickly. Average 43mpg
I can provide a copy of the excel if anyone wants to be as sad as me and keep track of everything?!