Gas Gauge drops to empty fast.

I went the other way new 16 gallon tank and sender. added 4 gallons and bent the arm (a lot) till it read 1/4 tank! Now it reads full a long time.
But in hindsight - I would have adjusted it to read an 1/8 with 4 gallons. FYI 4 gallons is just below the bottom of the hole for the sending unit