i've settled on 25s on my go-fast bike for most of the crappy roads around here (that happen to be the places i most like to ride), though it's as much because of my being "GS size", it makes sense to run a slightly larger tire as anything else.
25s might spin up a tad less quickly than 23s because of the rotating weight, but i can't say i notice a huge difference in rolling resistance. upside is they feel a bit cushier, and allow me to worry a little bit less about slamming the rims when ripping a heavily-textured descent.
i'm not unhappy with (high-quality) 23s and still have them for smoother situations, but 28s aren't an option as they wouldn't be able to fit in the CF fork.
on some bikes i've mixed slightly larger tires on the rear, but the current go-fast bike has crisp-enough geometry that i don't like the way it handles when the tires aren't a matched size.
the bikes i use for everything but go-fast generally don't wear anything smaller than 32s or larger. (the townie-fixie wears 42s; the lock-it-up-for-errands bike wears 1.95 'dual-sport-ish' tread.)
Originally Posted by pierce
do you run 28's on your training rides in the marin, sonoma back roads? I would.