I've been doing this for a little while now (though the car was off the road for a couple of months, so there's a hole in my figures) and it's quite interesting to see the trends.
I've been using a OO spreadsheet to track distance, fuel usage, cost, time between fillings, km/L etc, as well as the differences between values. Probably too many things, really, but it is interesting.
Here's the spreadsheet. Only seven entries so far, so the averages aren't the most accurate. And the numbers are all metric, and australian dollars. But it might be a handy template for someone else.
The EV bits on the right end are based on figures I've gotten around the intarwebs for comparison - one of these years I'll convert a car to electricity and test it out myself.
The file is .ods for Open Office, I can provide an .xls version if asked.
