Like most of my fellow CR listmembers I must admit that I have no special scientific information to add the discussion. What I will say is that I have tried 20 mm tires and hated them. I hate them for the same reason I hate extremely low profile car tires that have much less than 100 mm of rubber between the rim and the street. They are rediculous, uncomfortable, impractical and unattractive. When I have ridden 20 mm tires I have found that the rim is quick to dent due to road imperfections, and that the ride is horrible. I can't be fast when I'm avoiding every little nibblet in the road. I was so happy to see that years ago, Bicycling magazine published an article saying that rolling resistance goes up with the 20's. Now here comes the hard question ... Are 23's better than 25's or 28's? This is the only problem that remains for me because they are the only sizes that I believe ever look right on a race bike or for that matter a touring bike. Any wider and I think we are entering curb hopping mountain bike territory. The 30's also "feel" slower to me ... they make a different sound and they are heavy.
Here's my conclusion.: The 23's look about as nice as the 25's but the twenty fives have a better road feel. The 23's can be lighter so they give a lighter feel on acceleration. If I weighed 220 or more and I had to cope with bad roads, I'd choose the 28's. If I weighed 180 and up, even on spirited club rides, I'd choose the 25's. The really light guys should choose the 23's. They don't need the 25's unless the roads are bad. I also note that a road bike looks just as racy with 23's or 25's, so one should make their choice based on the road and their weight. The 20's look silly and should only be considered by the fringe set (the same fringe that puts 205/40/17 tires on their Honda Civic equipped with beer can exhaust tips). I wouldn't be surprised if the 25's turn out to be the fastest in practice, and faster than the 23's. The reverse wouldn't surprise me either. I would be very surprised if the 30's turned out to be the fastest. I am waiting for any conclusive double blind tests, but short of getting a robot to propel a bike down the road for a time trial, I suspect it's not coming anytime soon. Nothing in this post was designed or intended to offend, dismay or inflame.
Garth Libre in Miami Fl USA