Negative offset puts the hub closer to the vehicle, pushing the wheel farther away. Positive offset puts the hub further from the vehicle, pulling the wheel closer. Diameter of the wheel has no effect on how far a wheel sticks out.
9" wide wheel with a 0 offset puts the hub dead center so 4.5" backspacing (some places may say 5" because there's a 0.5" lip, but there are lips on both sides of the wheel). This is the distance from the hub to the vehicle side of the rim.
9" wide with +18 offset pushes the hub 18mm away from the vehicle, giving you a 4.5 + 18/25.4 = 5.21" backspacing. So there is almost 0.75" less wheel sticking out of the fender.
9" wide with -18 offset pulls the hub 18mm toward the vehicle, so a 3.8" backspace. It pushes the wheel roughly 0.75" farther out of the fender.
Typically you don't go much wider than 12.5" tire with a 9" rim because it can cause bulging of the sidewall. Running a +18mm wheel with a wider tire introduces the problem of hitting the control arms with the tire. I can almost guarantee you'll hit the control arms with a +18mm offset with anything wider than 295mm (~11.5" wide). OEM wheels depending on trim are 8" wide with +18 or 9" wide with +19.
Pic below is my Rebel with OEM fender flares, 18x9 0 offset wheels, OEM 275/70R18 Duratracs. The tread is even with the fender, sidewall is maybe 0.5" farther out.
View attachment 162288
