Are Wider Tires Faster?

There is some nuance on this topic that is often missed in the discussion around the question above. The TLDR; is that in general, a narrower tire is faster, up to a point, as a function of rim width & depth, because aerodynamics, unless you’re really slow.

Let’s say that I have a 25mm external rim. If I put a 25mm tire on it, best case it will be 100% of the rim width (25mm/25mm), but what you want is 95%. A 23mm tire generally solves that. Put a 28mm tire on that rim, and it doesn’t matter how deep it is because you’ve lost air attachment before the tire/rim junction.

The thing that most folks miss, and the cycling media/marketing types either don’t know or don’t clarify, is that while wider tires generally have lower rolling resistance at the same tire pressure (i.e. 25mm@85PSI has slightly more RR than 28mm@85PSI), no one uses them that way. You’re not going to ride your 28mm at 85PSI. You’re going to ride it at 70PSI, because comfort is why you put that tire on the bike in the first place. But even even if you did, that ignores aerodynamics.

Looking at the results over at BRR for their GP5000 S TR size comparison, the difference b/w a 25c and 28c tire at 80PSI is 0.5W in the 28C tire’s favor, comparing 25c@80PSI to 28c@73PSI, we’re talking about 0.1W. The same is true when making that comparison between the 25c@80PSI and a 30c@65PSI. At 58PSI, the 30C actually has higher RR than the 25c@80PSI…

Having an aerodynamic setup on these 25mm external rims, especially on the front, far outweighs 0.1W. How much it outweighs depends on the wheel depth. On a 25mm external 88mm deep rim the difference b/w a 23mm and 28mm tire is likely to be double digit…

Knowing all of this is why I put a 23mm tire on my front 25mm external 88m deep TT wheel, have shallow gravel wheels for my 42mm tires, and ride 25mm on my 28mm external 45mm deep daily drivers.

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