18x8 + 45 Front
18x8.5 +50 Rear

What is the widest tire size i can run on these rims?

Thanks

 

Depends on if you want to roll your fender at all, and what camber you run.
225/255 is good for those wheelwidths. If you are lowered, I would recommend a little fender flaring - I hope this doesnt scare you because it can help a lot and is easy/cheap!
Otherwise, I would expect a 215 in front with some aggressive camber would do the trick.