There probably is about 2 gallons left when it's "empty" but most of us fill up with about 15-16 gallons at that point but don't be alarmed since even when it's truly empty there's about a gallon of unuseable gas that sits slightly below the fuel pump.
I usually fillup after about 50 miles after the fuel light comes on.
Everyone goes so far with the empty light on, wow. I havent't run mine dry, but I've done like 25ish and at that point the top of the needle is almost below the lower end of the empty mark. Maybe you still have gas left once the needle is below the last line, I'm not sure.
I think city vs. highway driving is important also.
Ron makes a really good point. When you run out of gas the tank isn't fully empty. It's just that the level of fuel has fallen below the pump so it can't be pumped through the engine. I'm not sure exactly how much of the tank sits below the pump line but you won't ever be putting a full 18 gallons into the tank. I usually run mine around 50 miles after the light comes on and don't ever pump in much more than 16 gallons.
im not really trying to jack the thread, but since you kinda got your question answerd... how about mine:
how much gas do you pump in after the auto pump clicks it off? like i know people top it off, or round up to the next dollar, but i can put like 2 more gal. in it, what do you guys do?
I let it click once then I click one more time and I consider it full;you don't want to overfil since you need room for the gas to expand when it gets hot.
If you don't allow a little room for that expansion it put undue pressure on the whole fuel system and will saturate your charcoal canister and cause it to fail and throw a CEL and the fixx isn't worth the extra gas you squeeze in.
yes the light will come on at 13-15 gallons-but don't attempt to stretch it to the 17-18 gallon mark-as RonMaiden says about a gallon -or better yet round up to 2 gallons is "unusable" it will be below the fuel pump.
You'll only have to walk once for gas to learn.... I did, sputtered about 6 blocks from the gas station. I chose to park it and walk rather than attempt to run it dry to the bone.
Running on E once in a while is ok but running past E (fuel pump level and below) over a long period of time will start to damage your fuel pump and even injectors. Its as if you were running your truck lean...