I am running 18x8.5 +35 with 225/40/18 front and 255/35/18 bsck. The 35 is the how far the centre of the rim is pulled inside or ourside from the centre in mm.
TSW explaines here
http://www.tsw.com/explore/what-is-wheel-offset.php
The only trick with the offsets and rim/tire widths is you don't want to rub in stock spring on the inside or your outside fender.
And the only thing that makes a difference from the aspect ratio is it will throw your spedometer off. For example a 225/45/17 might be perfect in which it shows youre going 100km/hr and you are if you go to a 225/35/18 it shows 100km/hr but youre moving at 99km/hr. Its literally a joke difference.
Sent from my LG-H831 using Tapatalk