Why does my 1992 ranger get poor gas mileage?

I have a 1992 ford ranger custom, automatic , 2wd, 4.0l v6, and no cruise control. I replaced the plugs and wires, installed an electric fan and had a k&n filter. Even w/ the k&n panel filter my average was about 17.86 mpg( the k&n only improved the avg by 1mpg so I got my $50 back), and I only romp on it only when I need to p other than that Im light on the gas. I seafoamed it and put " engine restore 6cyl in the oil" . It dosnt have a rough idle. It has 165K miles. Im gonna be honest w/ you whenever I add those octane boosters it actually runs rough.