Gas tank telling lies?

[1970PAValiant]

my Valiant keeps going down, down, down on gas and when it says empty I go fill it.and it fills with 10.2 gallons in a 16 gallon tank..hmmm. so I did some searching of the internet and found this site, wanted to see what anyone thought

http://www.allpar.com/history/mopar/electrical2.html