Gas Gauge readings??

On my 72 demon the specs say I have a 16 gas tank, Well When I fill it up the gauge reads full. Now when It gets down to @1/8 tank I fill it and it only took 9.6 Gal. Could my gauge be that far off? The tank appears to standard it's not a fuel cell.

Thanks

You may have a float that's starting to take on gas. It may be the original float and tank unit. tmm