You can't normally use every drop of gas in the tank as the tank's bottom isn't shaped like a funnel. (standard saddle tanks are 17 gallon or 19 gallon, a 20 gallon tank would be unusual to find)
Then Ford should advertise that the gas tank is 17 gallons lah. My bike is listed as 3.5 liters but when it reaches empty I can pump in 3.7 liters.
Shall I sue Ford for not making a perfectly smooth gas tank that lets me use all of the gas in my tank?
No, but I'll would keep that that in mind for my next car purchase