I think the 23 is becoming the standard and the wheel is likely designed with the 23 in mind. That said, I have seen a lot of variance in sizing. One brand's 23 may be as wide as another brand's 25. If you are a big guy or the roads are really bad a 25 on the rear might be worth whatever aerodynamic penalty there may be. I don't think I would want a fatter tire up front unless I was on cobblestone.

http://velonews.competitor.com/...s-roll-faster_209888
Re: fatter tires
Dear Brad,
Very good question. As you’re probably aware, the aerodynamic data put forth by Hed and Zipp in support of wider rims for improved aerodynamics shows that it definitely depends on the rim. In general, if you ride very fast, the aerodynamic drag of the fatter tires will cost you more in speed than the rolling resistance benefit will gain you.
In an email, Zipp lead engineer Josh Poertner said:

In general, a wider tire of (the) same construction will have lower rolling resistance for exactly the reasons (you stated). Ironically, the best description and data on this comes from studies done in Britain in the 1800′s looking to optimize the width and diameter of wheels for locomotives. There is also a lot of great info related to this in “Bicycling Science” from MIT press, as well as Paul Van Valkenberg’s writing on racecar tires.
Generally, though, the decrease in rolling resistance becomes smaller as the tires get bigger. So for example, going from a 19mm to a 20mm may save 1 watt, from a 20mm to a 21mm may save 0.8 watt and from a 23mm to a 25mm may save 0.3 watt. There is great data on this in “Bicycling Science,” using old Avocet Fasgrip tires, which were available from 18-32mm. The 28mm and 32mm were nearly identical, but moving from 18mm to 25mm saved a few watts.
What they are missing is the aerodynamic piece. We have data from the Zipp 303 launch showing the 303 with different width tires (see graph). The figure tells the story of how you can really optimize for tires below a certain (width) number, but eventually the tire really dominates the airflow and ruins everything. In general, our wheels are optimized around 23mm tires, which means that 21mm tires usually run about equal, maybe a fraction of a watt faster, but don’t change the behavior of the wheel. Moving to a 25mm adds drag, but can also change the stall behavior of the wheel. And by the time you are at 27mm, you have something that behaves quite differently.
The question really needs to be in regards to the balance of lower Crr (coefficient of rolling resistance) from the wider tire against the aero penalty. The 303 was designed to be as good as possible with 23mm tires, and as a result, its rim is 28.5mm wide. To behave similarly with the 25mm, it would likely have to be at least 2mm wider. In the graph you see how the 25mm tire has the same curve shape as the 23mm tire on the X45 (code for 303FC clincher). The 27mm tire is on the 285FC (code for 303FC tubular), and you notice that not only is the drag higher, but the curve shape is completely different. In fact, the curve shape looks more like the Easton or Mavic. This is indicative of the rim not being able to clean up the dirty air behind the tire. Ultimately, the offset should be Crr watts vs. Aero watts. In this case you have grams of drag on the left; every nine grams is one watt, so from 23mm to 25mm, you have nearly no penalty up to 10 degrees, and then three-to-six watts at the higher yaw. With the 27mm, you have something like no penalty to five degrees, and then a five-to-eight watt penalty after that.
Ultimately for the Specialized I would say that the 0.2 watt (0.3 to 0.8 watt) of rolling resistance does not overcome the zero-to-six-watt aero penalty.
Last interesting note: we have been working with Jordan Rapp on this since he noticed that his ‘training Firecrest’ wheels with 25′s were ‘twitchy’ compared to his race wheels with 23′s… we thought this might be largely aerodynamic, but the shorter contact patch (you) discuss is actually the culprit; the longer contact patch serves to resist steering input and adds a slight damping effect to steering inputs. By lowering tire pressure to increase contact patch, the effect could be eliminated, even though the aero properties of the wheel remain the same.

― Lennard

On the newer, wider rims, a 25c tire is HUGE compared with a 23c. I train on 25c on some Hed Belgium rims and love it in the winter (wouldn't run Zipp FC for that anyway), but the combo might not fit on some frames anyway. I'd rec staying with 23c tires on the FC wheels for optimum aero/Crr/handling.

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Fatigue is biochemical, not biomechanical.
- Andrew Coggan, PhD

I'd go 22 myself. Even on those, narrow is aero.

This is a clincher 404 wheelset, right? I think I'd go with the Vittoria Open Corsa EVO Triathlon tire with latex tubes inside.

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http://bikeblather.blogspot.com/

Does this advice hold for the Hed Jet C2 clincher wheels, as well?

I'm running a Vittoria Open Corsa EVO Slick 23 on the front, and was considering running a 25 on the rear. Not that I've ever had an issue with a 23 rear. I just figured the slightly lower Crr may be a bit faster on the rear where aerodynamics are slightly less important.

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Steve

"If you ain't first, you're last." Reese Bobby Talladega Nights

You'd probably be surprised that Crr doesn't necessarily follow width as exactly as most assume...it's all about that "of equal constructions" thing. Even within a given tire model, that doesn't necessarily hold across different sizes.

2 of the fastest tires I've personally tested include a 22C and a 20C width...

But yeah, I would think that 22C Vittoria Triathlon would be fast on a C2 as well.

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http://bikeblather.blogspot.com/

Hello Tom - read your blog. Great work! Thanks for publishing.

Similar question to the one above: I am borrowing Zipp FC 404 and Zipp disk (both 2013) for an upcoming IM (Maryland). I wonder whether you would also recommend the Vittoria EVO Open Triathlon 22c's on the Zipp 404 / disk combo (Shiv bike) - for both wheels. Or would you use a wider Vittoria tire on the rear disk?

I used Michelin Pro4 Endurance on my own Reynolds Strikes 66 for a year +, and they were great. I took out the Service Course tires and got a flat on the first ride. (Same course I rode w/ the Endurance tires many times). I know it's bit of a bad luck, but when I looked at the side walls of the Endurance vs. Service Course tires, they just seem so different. Any idea whether the Vittoria EVO Triathlon has a better side wall protection than the Pro4 Service Course?

Thanks a lot!

Just as a point of reference - the zipp 303 with a 22m attack, 23mm 4000s, and 25mm 4000s:



Should be noted that the 303 is wider than your 404, and the 23mm 4000S is pretty big for a 23mm tire.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Thanks a lot. Would that suggest, that the 22mm will fit better the 404 front wheel? Would that be the same for the rear disk? Would you put the 22mm Vittoria Open Triathlon EVO even in the rear (Super-9 Carbon Clincher Disc)?

I would happily put that tire on the rear of a wide disc wheel. I would happily put a wider one on too. Hard to know which would be faster, wouldn't worry about it. The Vittoria Tri tire is a good rear tire, as long as it is fairly new (and thus not likely to flat)

I had my wife on 22m Attacks for her big TTs front and rear this year but would have happily run 24mm Force on the rear as well. Not sure what would be better overall crr vs aero.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

"Low Crr can make up for a lot of aero 'sins' "...especially for rear tire applications ;-)

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http://bikeblather.blogspot.com/

Many thanks for your swift responses Jack and Tom. I kept looking into this topic throughout the day, and have come to similar decision (mostly from Tom's tests and product reviews on slowtwitch). I think I will go w/ the Attack / Force solution (I like the theory behind stacking the tires and the 4000's these are based on have been battle tested for years). Also, the Conti's seem to have a sturdier side wall than the P4, and since I couldn't get my hands on the Vittoria EVO Tri, I think I will give them a pass (at least until there is bit more experience and user testing / reviews on the overall quality and performance).

Thanks again!