I reset the trip meter every time to zero, so I can see how many miles I do between each fill up. Enter all details on fuelly.com (i.e. distance (eg 178 miles); fuel purchased (eg 32 litres); price (eg 1.329 pounds per litre) and it calculates it all for you.
My 1.6 auto is so economical for a petrol auto I never bother calculating the m.p.g.
I've seen 50 mpg on the meter at times and that's good enough for me.
It's the depreciation cost that you want to worry about.
I did a comparison: trip computer verus actual mileage and fuel filled. The trip computer is off by at least 5% and errs on the side of optimism. I got 6.3 L/100km actual compared to 6.0 on the trip computer.
In that case I can forgive such a nice car a little thirst. I try not to obsess about the running costs but I have noticed that tyre life and replacement cost is 'on the wrong side of the fence'.
Incidentally, does any one know how to change the auto fluid? I fancy that a mid life change of fluid would work wonders for drive train life.
Just warm the fluid up, drain it out, replace bung, let the fluid cool, measure how much came out and add the exact same amount of fluid less 100ml (NISSAN ATF Matic D I think this is a dexron III, but it might not be, I know that Toyota use a proprietary synthetic fluid that costs £10 a litre).
Then check the level, and hopefully you only need add the 100ml you kept short.
If you want to invite trouble, you can drop the transmission oil pan, and remove and clean the "filter" which is a simple gauze strainer. Replace with new gaskets.
Nissan reckon the fluid only needs "inspecting" every year, and will last over 120,000 km (as Clarkson says, that is nearly 500 miles