35 might be the official number this is why I said "squeeze". Personally I would be slightly put off by these numbers. I guess it depends on the kind of gravel and fire roads you ride. It's easy to put narrower tires on a bike with more clearance than the other way around. When you say the FX S4 is 2kg lighter is it comparing to the Toughroad or to the FX3? The S4 is one sexy bike but the Toughroad might give you more versatility should you want to ride some rougher stuff.