Originally Posted by
oldbobcat
Depends on the surface being ridden upon. These days, for general road racing, 25 mm is the norm, with wider tires used for rougher roads (like Paris-Roubaix) and gravel (like Strade Bianche). The larger contact patch reduces friction between the tread and the road, and the lower pressures let the tire roll rather than bounce over the texture of the pavement.
Minor point - they never go really fast at Paris-Roubaix, even on the pavement. The route is essentially flat. Now I did some descents in Cycle Oregon three weeks ago that were fast enough for tire width to make a big difference. Close to 50 mph. Aero rules at those speeds.
And yes, narrow is the rule on older race bikes. My Pro Miyata ('83) takes 24 max. Never measured the Fuji Pro I raced in the '70s but I could not get a finger between the tire and seattube, even with Criterium Setas. Of course, non of us knew what width tires we were riding. I could, with full contact with the seattube, just get my cotton training tubular wheels in the horizontal dropout inflated. Rides on both of those bikes were/are superb with decent tubulars that are way too skinny for this age.