Not sure about small trucks, but my big trucks (Freightliners & Kenworths) get about 250 to 300 miles per gallon I think. I would think that your truck should have a gauge so that you can watch it. My GMC is a gas job, but you can probably scroll through the menu on the dash board and see how much you have. Like Saddlesore said, I wouldn't worry about keeping it full because I've heard that stuff can go bad also. However, on big trucks, they recommend keeping it on the full side because you can get crystal growth inside the tank. Not sure what to tell ya about that. One thing for sure is that you don't need to keep it in the back of your truck. Remember, it's piss water and it'll freeze. Most truck stops have DEF pumps now and you can buy it at any automotive store.
