Originally Posted by
McBTC
The 25s save a couple of watts due to reduced rolling resistance (if you can pedal 18 mph at the same psi as the 23s) but the 23s are saving up to 9 watts when going against the wind
It's all about the tire-rim combo, the intrinsic effects of a 2mm difference are miniscule. In reasonable circumstances, the only way that there's going to be a 9W aero diffence between a 23 and a 25 is if the rim was designed to be aerodynamically paired with a 23mm tire, and the 25mm tire crosses a width threshold creating an aerodynamically poorly-behaved pairing. Paired with an appropriate rim designed for 25mm tires, there should be no such behavior.