Just saw your reply when I saw your other thread with the poll.

To answer your question, no, mine doesn't do that. When the light comes on, I have between 35 and 50 miles before I'm out. The light comes on at empty on the gauge. When I then fill it, it will take 17+ gallons, as it should, with a 19 gallon tank.

First off, the Nissan guys you're talking to are idiots. There is no magical 5 gallons that sits in some mystical area for it to go bad with age and the truck (not car) will never use. Believing that is non-sense. They probably told you that because either they believe it (which they probably don't), in which case proves they are idiots, or they know better and don't want to bother with your issue knowing that it's still under warranty so they won't get much for the work. So they told you that to see if you'd believe them and go away. Which it appears you didn't because you're here on XOC for some real answers.

My suggestion would be to find another dealer/service tech and ask them about it. If they then try to push you off as well, make it case by not believing their BS. If you have a 20 gallon tank and you run it to empty, it should take close to 20 gallons to fill it back up. Pure and simple.