Each time I get gas I write in a little book, the following info.

Date
Station Name
Station Location
Price Per Gallon
Total Cost
Total Number of Gallons
Odometer
Trip Odometer (reset at each fill-up)
Type of Gas

I also note the date and mileage of my mods.

I decided to enter this all into a spreadsheet and see what numbers I come out with. Here is the summary.

Average MPG:
Stock w/ Dino Oil: 15.5218234
Lift w/ Dino Oil: 14.5080771
Lift, Front Bumper w/ Dino Oil: 14.5002732
Lift, Front Bumper w/ Synthetic Oil: 14.2961943 *This shocked me
Lift, Front Bumper, Tires w/ Synthetic Oil (I used 3.7% for the change in tire size): 13.0536942

Yearly Stats (7/14/05 – 7/11/06):
Total Miles: 22145
Gallons of Gas: 1495.753
Average Price Per Gallon: 2.490816327
Total Cost of Gas: 3728.67
Average MPG: 14.80525193
Best MPG (between fill-ups): 18.0507678 (hi-way from Pittsburgh to Detroit)
Worst MPG (between fill-ups): 10.6271658 (weekend of wheeling)