the 17s are 8'. I had dr's on my 17s that were 275/40/17s but i did not use those tires for a DD. like he said, 245s really is the biggest I personally would do but i think some guys are doing 255s.You're better off buying a better tire at the right width than a wider tire.
I'm not sure what size the rims on the S197s are, if they're 8" I wouldn't go over a 255 (others will say 275), if they're 9" I wouldn't go over 275 (others will say 305 or 315, depending on the tire).
I'd do 255/50/17 nitto's on the stockers.
I agree with Juco. The 255s will be fine especially Nittos.
:withhomos255/50/17 nitto's on the stockers
Your welcome. Sorry nobody else had the same advice.