I rode on a Conti sponsored team back in the 90’s and I remember been given 18mm clinchers for racing. I believed at the time we had an advantage over the competition. 18mm was more aero, ridden at a higher pressure and had a pin sized contact patch. It had to be faster. No idea that some 20+ years and a lot or miles later I would be favoring 25mm tires over 23mm and not even considering using anything close to 18mm. What’s changed?
Mostly research vs reasoning. As the market has gown tire companies have put more into bike tires and invested newer technologies with the hope of beating out the competition. It’s unanimous amongst manufacturers that 25mm is faster than 23 or 21mm over all terrain for all riders regardless of weight and road conditions. The big driver been the shape of the contact patch. Larger tires at the same pressure have a more round and not as elongated contact patch. Meaning less of the tire casing flexes as the tire rolls. Also allows for the same feel at lower pressure (approx. 10-12psi on 25mm over 23mm) and the tire at lower pressure rolls better over small bumps in the road. The shape of the contact patch also has more grip and feels better in corners. Literally everything about the larger tire makes it faster, more pleasurable to ride and safer with the only drawback been a slight weight advantage for the smaller tire (aprox 15g 25mm vs 23mm)
Aerodynamics come into play as well but this has mostly to do with the relationship between the tire and rim and not the 2-3mm difference in tire size. Eg. Using one of the new style wider carbon rims with a wider tire that inflates at the same size is optimal, using a 25mm tire on an older style 19mm rim is aerodynamically not as advantageous but you still get the advantage of the larger tire.
So until we say otherwise 25mm is better than 23mm in almost all cases. Unless you just don’t like the look of the fatter tires and you want 23’s or 21’s. Then looks surpass performance and you are still going to enjoy your ride (just not as much)
