What offset for 17" rims and 305 or 315 tires?
 

I'm trying to figure out what offset I will need for a 17" rim to clear the trailing arms/suspension, but still allow me to run a 305- or 315- series tire. This would be with stock rear fenders, definitely a roll, but I want to avoid pulling the stock metal. I know it's possible; I had a friend who had an FD years ago and was running 315's on a 17" Fiske or Rega wheel. Unfortunately he doesn't remember the offset at all. I know there's a very narrow margin for the correct offset with a 17" rim and that large of a tire.

PS - this is for the rear. I plan to run a 275 series up front. The reason for all of this is I will need the traction, and the tire sizes for a 17" rim IMO are much more supportive. :) Thanks!