Well, if the 303 data is any indication, it'll be slower(and I believe the 303s were designed to be MORE forgiving for 25mm tires), but for the riding you're describing, I'd probably ride em as they are and then swap em out for 23s when they're done.

P.S. the difference likely won't be noticeable.

http://velonews.competitor.com/...erodynamics-Zipp.png

For road racing 25 is a reasonable enough option to not be worth replacing them.

When they wear out I would go with 23 though, at least up front.

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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

Agree with the posts above. The 25c Vittorias should provide an awesome ride for almost all road racing and fast group riding situations. For purely timed events I'd drop down to 23c at least in the front but the current setup is a very fast, low Crr setup with a big cornering patch and if you don't pump them up crazy hard they should provide a very plush ride.

Wear out the 25c Vittorias and if you plan to TT on these wheels consider a 23c up front as a replacement.

-Dave

If you have access to a vernier caliper or similar measuring device, I would suggest you inflate both the 303's and 404's to riding pressure and then measure tire width. With our Zipp 303 FC clinchers (on road bikes) with latex tubes, I found that Vittoria Open Corsa Evo CX tires in 23mm width, measured closer ~25 (24.9mm) and 25's measured closer to 28mm (~27.8mm). While the 25's offered a really nice ride (inflated to 100/105), even before I measured the tire widths, on group rides I couldn't shake the sensation that rolling resistance was a tad bit high thus went back to riding the 23's.

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Interesting - never considered the tire specs could be off that much so will definitely measure and report back. In terms of rolling resistance I guess I mistakenly assumed the wider tire would be lower but would give up weight and be less aero. I am on an R3 if it matters.

If you're not doing TT's, I would say you'll have a hard time finding a nicer ride. Enjoy!

ps. I heard somewhere (Fleck) that 25 is the new 23!

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/kj

http://kjmcawesome.tumblr.com/

wider tires Do have less rolling resistance, all else being, equally, and inflated appropriately. You can run a little bit less pressure the wider the tire is, for optimal performance.

the NET resistance to forward motion will be higher sometimes though, due to increase aerodynamic drag.

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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

Time-wise...i doubt you'll see much of a difference...comfort-wise...you'll see a noticeable difference.

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Cervelo S2/Zipp 404

I can't see the reason why not? Comfy..fast...neglible weigh differance.