I'm running 215's with no issues. IPD's website says you can also run 225's on Gen 1s. Not necessarily agreeing; just sayin'. I wouldn't go any less than 215's, and would seriously consider 225's next time, myself.
I run 215/45/17 on mine.<p>225/45/17 is the closest to the stock 205/55/16 or whatever 15" tire size Volvo used but I've seen a couple of 70s have rubbing with that size and a bunch that do not. <p>215s only throw the speedo off by 1% which is pretty minimal.