Originally Posted by
RChung
There are (at least) three four things to consider:
1. Which tire matches your rims best?
2. How fast do you expect to ride?
3. How rough/smooth do you expect the road surface to be?
4. Tires that don't fit inside the frame (and caliper brakes) are always slower.
I would think you'd want to use a 28 on the rear. I suspect (but don't know) that if the road is smooth, you're fast, and you have the right front rim, a 25 might be marginally faster than a 28. But because having to keep two different sizes of tire is kind of a pain, and sometimes the road isn't smooth, I might run 28 on the front.
Added that fourth thing. A 25c tubular (that measured out exactly inflated to 110 psi brand new) came to a stop in one revolution; bike on the stand. My Pro Miyata with a very short Cyclone sidepull brake. Tire rubbed the bottom of the calipers (but cleared the brake bridge).
I am a fan of running the same or larger tires in front. 1) road adhesion issues and resulting crashes are nearly always "prettier" when it is the rear wheel that slides out. 2) the racing adage I heard many times in my racing days - you want your fastest tire in back. On that Pro Miyata, I run 25c front, 23c rear. Love it. (And pay more attention to parallel cracks in the pavement than I do on my other bikes.)
Edit: I don't time trial. Yes, that bigger in front is aero slower. And again, those words of those club vets when I started racing - crashes are slow. Both in that race/ride and they too often impact entire seasons. I chose tires with that in mind. (And love that Vittoria has brought back that sweet ribbed tread I took so for granted that I never thought about it until it disappeared around 35 years ago.)