Originally Posted by
rm -rf
It's only 2mm wider.
But a 25c tire has 18% more air volume than a 23c. Not all tires actually measure 23mm or 25mm, but this is a significant difference.
So it's easy to use 10 psi less pressure in the larger tire and get a more comfortable ride, with just as good or better rolling resistance. It really shows on rough roads, like chip-seal gravel over tar roads.
The math. (The 18% increase surprised me. )
23mm diameter circle is 415 mm sq area.
25mm diameter circle is 491 mm sq.
I wouldn't call 18% a
significant difference. Yes, the 25 mm tire has a larger cross sectional area but if you laid the circles on top of each other, they wouldn't appear that much different. 2mm is slightly larger than a dime is thick. Even taking into account, as you've noted, the variance of the width of tires, going from a thin 23mm to a thick 25 mm, there wouldn't be that much difference.
One thing I have noticed in tires is that narrow tires like a 23mm or 25mm or even a 28mm tire tend to run narrower than the value on the sidewall while wider tires like 32mm, 35mm and 37mm tend to run wider than the sidewall value. I have a set of Vittoria Hypers that were advertised as 35mm but they are clearly marked as 622x37.