Probably more likey a problem with either the gas gauge than the gas tank - unless you put a 5 gallon sized dent in your gas tank !
Not a good idea. The gasoline serves as a coolant for the fuel pump, and running out can shorten the live of your fuel pump - mine failed in Va on a trip back from Florida to NY back in 1998. Wasn't very much fun.Roesterman said:Or tank still has 5 gal in it when you filled. Try running right out. (bring a 1 or 2 gal can of gas)..then fill up after you have run out and added the 1 or 2 gal.