One of the results of having a father as an engineer is that I have grown up taking stats for each gas fill-up.
Each time we get gas, in any vehicle, we record the date, gallons, price, odometer reading, and mpg for that tank, in the little "gas book." Miss a data entry, and you've thrown off the whole year.
Subsequently, I have taken this data-gathering habit over to my own vehicle, and after many incomplete years of data entries, I have stayed up-to-date for 2007. Although I need gas today, this is what I've come up with for the year so far:
Total miles: Over 8,181
Total number of fill-ups: 24
Total gallons: 327.125
Average gallons per fill-up: 13.6
Total amount spent: $956.18
Average amount spent per fill-up: $39.84
Average price paid per gallon: $2.92
Average miles per gallon (based off each fill-up's average): 26.2mpg
Best tank (average mpg): 30.6mpg (HAVOC HAVASU!)
Worst tank (average mpg): 22.8mpg
Most expensive fill-up: $47.66 (last time I got gas!)