it kinda depends on the design of the rim as to whether or not the caliper will fit.
+42 will be fine. it wont be too wide. if anything it will be more tucked in than a +40 offset. i have +45 offset for 7" wide rims and it sits perfect.
the offset is the distance away from the middle of the wheel to where the wheel makes contact with the hub. so basically the wider a wheel is the more positive offset you want to stop it from hanging out from the guard.
Bookmarks