There are many factors that determine your individual miles per gallon. The ONLY thing we have in common is we have the same truck. Factors that are different but must be taken into consideration are: Driving Style, Geographic Location, Fill Up Habits, Tire Type and Pressure, Loaded Weight, etc. Now, for all of the above factors there are many factors that will cause your individual gas milage to vary, such as: Ambient Temp, Geography, Tire Pressure, City or Highway, Wind Load, etc.
Now with all of these above conditions it should be impossible to determine if your truck you drive in Texas gets better or worse gas milage than the one driven in Colorado or Ohio or California...right? In my truck, I have gotten between 14.7 and 23.3 mpg!!! Quite a range but considering what I was doing when I got the above figures I know they are correct. If you are really serious about checking your gas milage and comparing it to other members of this board, here's what I suggest you do:
1. The participating people should have a standard for filling the tank. Fill to the first click and thats it. There will be a minor deviation every fill up but thats OK. It will even out over time.
2. Record milage at fill up and gallons required to fill.
3. Record date
4. Make a very short note of type of driving during last tank of gas. Interstate, city, heavy load, trailer, etc.
5. Divide the miles traveled by the gallons used and record it. This will be your AVG MPG.
5. Record data in a spreadsheet program or do it long hand, it does not matter.
OK, that takes care of the data gathering stage. Next comes the calculating stage. I fill up an average of once a week, but the time between fill ups doesn't matter much. Wait until you get 12 fill ups before doing this calculation. If you have already have 12 recorded fill ups, you can begin!
1. Add your previous 12 AVG MPG figures and then divide by 12. This will give you an average miles per gallon over your last 12 fill ups.
2. Enter your next fill up and calculate your AVG MPG and enter it. Drop out the 13th one back and pick up the last one you just entered. This should give you a new average.
3. Repeat every fill up.
Because of differing factors such as tire pressure, ambient temp, highway or city driving etc, you really can't look at one or two tankfuls and determine anything. By adding up your previous 12 AVG MPG and dividing by 12, you will take most of those factors out of the equation because you are now looking at averages over time. This formula is called a Trailing 12 AVG MPG. You could do it over a shorter or longer cycle but the longer the cycle, the smoother your curve will be. I picked 12 because if I fill up once a week then I will be looking at a 3 month running average.
I know this is much more complex than dividing your miles traveled by the gallons used, but if you are serious about comparing your truck against someone elses and you want to take as many variables out of the equation as possible, this is one way to do it. Hope this helps.