https://www.rsracing.com/tech-wheel.html
Positive offset is how far the hub surface of the wheel is to the outside of the car, in relation to the center line of the rim. Negative offset (a "deep dish" wheel) is rare and usually found on older RWD cars or trucks.
An offset difference of 3 mm is not enough to worry about. The 48 will fit fine..... A 3 mm increase in offset (a larger number) means the inner wheel lip will be 3 mm closer to the strut and the outer rim will be 3 mm further away from the fender lip.
Less offset (a smaller number) means the rim sticks out further toward the lip of the fender. You can't put a 40 mm offset wheel on and add a spacer to make it fit better - the extra thickness of the spacer would push it further away from the hub and make it WORSE.
The only time I would worry about having the wheel and tire a little closer to the strut is if I ever planned on installing snow cables, but Subaru recommends against that anyway.
The 40 should fit fine too, and probably look a little better. Anytime you put wheels on that have a different offset from the OEM wheels, it will affect the feel of the handling and steering. For that reason it would be better to stay as close as possible to 45 unless there is a good reason to change.
I think it is wise with any non-standard wheel to mount one tire only, and then install the wheel and tire on the front of the car to check for fit and any problems like rubbing at full steering lock (hard on the steering stop) and hitting the fender lip under heavy spring compression. You can lift the tire as high as possible using a floor jack to check. If everything looks good you can go ahead and mount the other tires.
IMHO, neither of those choices will be a problem, but always check the fit!
BTW your English is fine, and welcome to the forums.
John Davies
Spokane WA USA
