Every time I fill up, I write the odometer reading on the receipt. Then I enter this and  the date, total cost and number of gallons on an Excel spreadsheet for a record on a yearly basis. This gives an accurate mpg,  and you can also extrapolate to get some other interesting information, e.g. cost/mile, miles/day, cumulative mpg and miles per day; and projected yearly figures like total miles, gallons and cost. And if your driving habits are like most people and you plot the number of days between fill ups vs. mpg between fill ups, you'll see the inverse correlation between the two.
You'll also be able to show the wife or husband, how your driving habits produce a better mpg than theirs!

Frank, you're even more compulsive than me.  

FrankM: I too use an Excel spreadsheet, and have for all my cars dating back to 1992.  I enter the same information as you and get about the same list of calculated results, so our spredsheet designs must be similar.  Of course if you set this up for one car just once, it's easy to copy the spreadsheet and erase the old data to build records for another car.  Here are the figures for my automatic transmission LS2 '07 convertible with a 2.56:1 axle ratio, updated from what I posted back in January.

To honu and tgstoner:
Honu: My wife uses another descriptive before "compulsive" that begins with an "a". It's been a curse of mine throughout life. But I do remember the VW Westphalia Camper, although my preference for new wheels several years out of school was a '63 Pontiac Catalina convertible.
The spreadsheet with gasoline usage and mpg is pretty interesting, but I find that a separate spreadsheet with the dozen or so maintenance items to be of greater value - things like fluid flushes, air and fuel filters, serpentine belt age, etc. Even though you have all this info with the receipts in your maintenance folder, the spreadsheet allows you to see it all at a glance with notes or items highlighted on maintenance to be performed with the next oil change.

Tgstoner: My figures are about 2 to 2.5 mpg greater than yours because I have the 6-speed manual xmission. I also drive pretty conservatively except when challenged or when I "pick my spots". Even though my Z51 xmission has taller gear ratios in the first four gears, their effect is probably negligible when driving conservatively.  To avoid the 1 - 4 forced shift when driving moderately, I ordered and installed a CAGS bypass the very first week after deliveryof my C06. My driving and shifting enjoyment tripled when I no longer had to worry about being blocked from second gear. It was difficult not to instinctively try to "mash" into second when it was blocked.

I have an '07 Z06 and I've gotten as high as 26 mpg on a cruise to Vegas from the SF Bay Area.  On the way back... fuel mileage was not a concern.  I was just enjoying the desert vistas at 100+ mph for a good portion of the drive.    However I have seen the instant reading hit 40 mph a few times.  Around town I generally get about 17-19 mpg depending on whether or not I've spent any time in traffic. 

Yeah, but the instant mpg doesn't mean beans.  I've seen it run at 99 for a while when coasting down a long hill.  Apparently it won't go to triple digits.  Of course if I'm going hard up the same hill, it'll say 8 to 10 at best.  And then there's the instant on the drag strip.  :-)

The car computer is instaneous mpg and does not truely represent mileage.

Here is an update to the figures I posted back in January, 2010.  We took the car on two 8-day trips in July, 2010, for a total of more than 6,500 miles and generally running 5 to 8 miles per hour above the posted limit we were experiencing exceptional economy.  This is a great grand touring car! Greatest distance on a tank: 481.0 miles (7/7/2010; 16.604 gallons; 28.969 mpg)