What has been said will give your mileage for that tank of gas.. which is usually more than enough for most of us. If you want to go that extra mile, do this across more than one tank.. or better yet, keep a fuel log in the truck at all times. Record the amount of fuel pumped (gallons) and the odometer reading every time you fill up. Then you can compute the averages over time or the total mileage between any two fillups (sum of miles driven / sum of gallons pumped for all fillups between). I usually write down the total dollar amount of the sale and the date so I can track fuel costs as well.
Every now and then I enter the data from the log-book into an excel spreadsheet and let it crunch all kinds of interesting statistics for me. I even posted that spreadsheet
here on RF.com.
I've been called obsessive about this though..