Originally Posted by
Carbon Unit
I have used 23 and 25MM and I don't really notice much of a difference. Maybe if I tried them back to back I could but I switched from 23s to 25s because the 23s go on sale and are more easily found.
The difference is a couple watts per tire. Of course you couldn't feel the difference.
The current issue of Velonews does a test of tubulars. While they aren't clinchers, the points cover the same which mean people can't generalize about tires, sizes, and air pressure. A lot boils down to how specific manufactures construct their tires/casings and the actual conditions. Some tires are faster in wider sizes, a few aren't. It's all about maintaining contact area, especially when the tire hits road imperfections. Some tires are more supple, which makes for a smoother ride but adds to the rolling resistance. Adding too much air pressure generally increases rolling resistance, but not necessarily. VN tests showed on smooth roads where the tire doesn't bounce, higher air pressure is faster.
So saying 25's are faster than 23s for all tires isn't always true. The same with higher air pressure.