Originally Posted by Rides4Beer

I run 23mm GP4Ks on my 28mm LB rims (they're 21mm internal, which is the important part for tire fitment), they measure out to 27mm, perfect aero configuration. 25 or 28mm tires would work too, if you want a lil more comfort. You really only give up a few watts. I'm learning with my gravel bike on 28mm GP5Ks (that measure at 30mm on 20c wheels), that comfort can actually equal speed if your roads are rough.