I once put 700 x 25 Continental tires on a FX. A few year laters, I look back and noted that it did not help make me faster. Made the ride uncomfortable! Fast forward, on my kids FX, I kept the 700 x 35 tires and recently replaced the front with Panracer due to a bad tire. Happy with the 700 x 35s. Will be replace the rear tire eventually.