Slant Six Fuel Mileage?

I fueled up 45 miles back and the fuel gauge has dropped from "F" to 3/4 tank? The original owner's manual says this car has a 16 gallon tank, I guess that is correct? We installed a new sending unit 10 days ago and it just seems to be a bit hard on gas. I'll top off Saturday and see what the math says about the mpg on my /6 engine. It just seems like that gauge is moving south in a hurry? Checked for leaks at the new sending unit and it's still dry? Thanks in advance for your input!

Mike