My rear fender is clearanced for a 18x9.5 wheel with 65mm offset and 285 35 18 tires, im just wondering how much more clearancing would be needed fo 17x11 wheels with 50mm offset with 315 35 17 tires?

 

I also have a '94 Formula with 17x11 wheels. I'm using the 36mm offset and only had to roll my fender lips to clear the tires, even though I'm lowered almost 2" in the rear. If you're using a 50mm offset you likely will not have to do this, but do a trial fit to make sure. What you will almost certainly have to do is some hitting with a hammer on your inner wheel well. You may also need an adjustable panhard rod if you've lowered your car. When you lower your car the rear end is likely no longer centered.

 

Sounds like to me you have a VERY inside offset. Roughly, 12-13mm is 1/2".

Like Mike said, you really shouldn't have a bit of trouble but to be safe, you might roll the lip of the wheel well up to make sure you don't hit the tire on hard bumps. I use 50mm offset ZR-1 wheel with a 1/4" spacer and they fit about as perfect as you could get.