Originally Posted by smashndash

Don’t worry about that too much unless you’re doing a TT or something. The difference in drag between a 25mm tire and a 28mm tire will be a couple watts at 30mph on the 56 or even 46 rim, assuming you go for a narrower 28 like the GP5k or Schwalbe Pro One Addix. Even a Vittoria Corsa 28 will be fine. If you want a 28mm tire, get a 28mm tire.

That being said, if you have rim brakes, that might be tough. The new gen Shimano brakes have (almost) no trouble fitting the 56 rims (the brake pads will initially be at an angle), and definitely no issues with the 46 rims. A narrower 28 on the 46 will be doable, a narrower 28 on the 56 will be cutting it close and a fatter 28 on the 56 will probably be too much, depending on how your brake is mounted.

What bike do you have? What is the stated tire clearance for it? Usually rears have more clearance so you could go for a 25/28 combo, which is the best of both worlds.