It's compulsive, I admit, but I have kept a log book of every car I've owned. Every drop of gas, the miles, the cost of gas, the oil changes, the new tires, the type of gas.....everything*.
And I used to enter it into a spreadsheet and do the stats. MPG is not a steady state, to understand it you have to do some moving averages, adjusting for various variables. But it averages out over time, and the error in data, such as poor calibration of gas pumps, is at least smoothed out. It's good to examine $/mile as well.
The hybrid just tells me the mpg on the cute little screen, and although I keep the log, I don't create the spreadsheet anymore. But that number on the screen (heck, it's a little graphic histogram) it keeps me paying attention to the way I drive.
(*What a geek!)
Data.