Tire Size Reëxamined

Like that umlaut? The Economist does it, so I figure it's probably correct.  Anyhoo... all of this hoo-ha about tire sizes and inflation and tire size versus aerodynamic speed has had us thinking about this quite a little bit lately.  

Step 1 was to to further examine the effects of tire size on aerodynamics.  Ideally, we would have been able to naturally (i.e. without artificially manipulating bead seat width) set two of the same model of tires up to the same inflated dimensions on wheels with different bead seat widths, to test the slope that we'd earlier established that had the 404 and 52 leap frogging each other at inflated width. The 52 with a 23 is, inflated size-wise, a bit smaller than a 404 with a 25 and moreso bigger than a 404 with a 23.  It is a little closer in speed to a 404 with a 23.  What would happen if you put the same tire inflated to the exact same dimensions on a 404 and a 52?  Unfortunately, that's an unanswerable question, but we were able to test a good approximation.  

Continental makes the Attack 22.  It's a different tire than the 4000s II, but its inflated dimension on a 52 closely replicates the inflated size of the 404 with a 4000s II 23 - the Attack 22 on a Rail is .2mm narrower than the GP4000s II on a 404, and about .6mm shorter than the GP4000s II on the 404.  There are tread differences, which are known to influence aerodynamic speed, but net of everything it's the closest we could get to being able to measure exactly what we wanted to.  This was part of a totally separate trip, primarily to do some totally different testing which we can't yet talk about, and we shoe-horned this bit in.  We ran the 52 first with the GP4000s II, and then with the Attack 22.  With this testing reaching the point of diminishing returns and threatening to turn into a bottomless money pit, we kept it to just the two runs.  Since we don't directly compare different tests from different days, we used the Rail 52 with 23mm GP4000s II as the baseline, rather than the Pacenti SL23 which was the baseline for the other round of tests.  Semantics, we know, but it counts.  



As predicted, the narrower tire gains some speed.  Whether it's the same speed gain we'd get from the theoretical GP4000s II that set up to the same dimension that the Attack 22 set up to, we can't say.  We're getting well into the realm of splitting hairs here.  

So now, the million dollar question: which tires should you be using?  We've now examined the relationship between inflated dimensions and effective pressure, and the relationship between aerodynamic speed and tire dimension, and the fact that people don't want to ride around on the road at 60psi, and a few things show up.  The first is that you should ride the tires that you like, at the pressure that you like.  There's no "right answer."  That said, it seems to make little sense that a 110 pound triathlete should choose a 25, on which she'd have to knock the pressure down pretty low to get a smooth ride (also taking into account the rims on which she's mounting them), when she could instead get more aerodynamic speed, better protection against pinch flats, and a comfortable and secure ride on a 22.  Conversely, a 200 pound guy doing performance recreational rides - group rides, gran fondos, etc - on beat up roads can choose a bigger tire to advance his priorities.  

Varying tire size, type, and inflation pressure gives you an array of tools to tailor what's between you and the road more specifically to your priorities.  Rather than try and drill down into making you feel as though there's a specific answer of what, how much, and how wide, what we've tried to do is to give you the tools and information to help you discover your own perfect ride.  

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It's definitely a factor, and there are several sites where measurements are published, but we have developed our own machine to measure and will be starting that up in the coming week.


I am sure I am not the first to mention this, but seems like rolling resistance should factor into this equation too? I realize that isn't something you were measuring — at least not something for which you have published measurements. I know that in theory at the same pressure (and that is probably a key point) the wider tires should perform better. Now …how that trades off with aerodynamic costs or comfort … ?


Technically, it's a diaeresis, not an umlaut.


Hi Dave CIf my math is correct, it sounds like you are at around 105 rear and 90 front. I'm going to premise this by saying that if that works for you, then it's right. Since you are using a bigger tire on the rear, I would expect there to be almost no psi difference between front and rear. Since you are on a TT bike, your on-bike weight distribution is much more front-centric than mine on my road bike. That would also favor a more even front to back pressure distribution. So there are some things that don't necessarily line up for me with your formula, especially since it doesn't take tire size (stated or inflated) into account, but if it's allowed you to get to a good value for yourself, go with it. Side note, since I've had this inflation pressure bug up my butt so hard I've been REALLY diligent about checking and correcting tire psi before every single ride. I have a compressor and a prestaflator (everyone should have one, I'm actually angry at myself for not having gotten one sooner – I've had the compressor for years but the prestaflator makes it worth having) and cross-check with an SKS gauge. Oddly, I've found that on my current wheels/tires (Rail 34, Michelin Pro4 SC 23mm), I prefer a touch higher pressure than I'd thought I did. I'm at 86 front, 92 rear. It's not jarring, gives great traction, feels awfully speedy, and I like it. Conversely, my mtb pressures have gone ridiculously low (for me) – currently 20 front, 23 rear, and I don't think I'm done lowering them. I use huge tires (29×2.3 stated front, 29×2.25 stated rear) and it's actually made me think I might prefer smaller tires, because what are you going to use – 18? Ambient atmosphere is 14, which is another way of saying a flat tire has 14 psi. Cross tires are low and lowering, but I am also on tubeless for my first dedicated go of it, so it's not as straightforward as the other two are.

Dave Kirkpatrick

Very interesting topic and one I have been researching for several days now. The dymanics involved here of weight (both human and machine), tire width, rim width, etc. boggle the mind. My greedy question is to find the optimal tire and pressure combo that satisifies several parameters (by priority): comfort at century distance, minimal risk of pinch flat, and speed. At present, I am considering, on TT bike, of running GP-4000s, 23F, 25R, but unsure of the correct pressures. I weigh 160-lbs. I have been using the formula Rear=0.33x(body wt)+53, Front = 0.9x(Rear). I don't recall where I found that but it's my starting point. Any recommendation and whether I am even on the right track? BTW: wheelset is Pacenti SL23.


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