i love using excel to do all the work for me.
heres what i do:
lets start at line 5
A5 is where i enter the date
B5 is my calculated gas mileage
-enter " =D5/F5 " without the quotes
C5 is where i enter my trip OD miles for that tank
D5 is my "miles adjusted for tire size" formula (takes info from C5, its also the cell used for B5)
E5 calculates my cost per gallon (to keep from having to enter it myself every time)
-enter " =G5/F5 "
F5 is where i enter my gallons pumped
G5 is where i enter total fill-up cost
H5 calculates "cost per mile" for me.
-enter " =G5/D5 "
I5 is for notes
if you don't have mileage correction, just skip that one, and move your formulas over accordingly.
once you enter the formulas a few times (remember to change the number to whatever line you are on, and letters stay the same), excel recognizes the pattern and fills in for you.
i also did different width borders, put the mpg in bold text, and some other little things. i need to print it out and scan it as a .jpg, so i can post it up as a picture.
once its all set up, and remembers the pattern, keeping up with it is a piece of cake. i just fill-up, enter miles on the receipt, enter info into my computer, and throw the receipt into the recycle bin at work.
btw, the "cost per mile" has been really informative, almost more than mpg. when gas prices hit their summer/fall peak, i was getting my best mpg, and it was costing me .20 a mile to drive. now that the weather has gotten colder, my mpg have fallen. but so have gas prices. now i'm paying .10 a mile