That photo makes me miss being within a short drive of Mt. Baldy. Even if thatās not Mt. Baldy.
To conform with the Rule of 105 you would have to run a 25mm tire. The data Iāve seen for rolling resistance between similar 25mm and 28mm tires at 100psi is about 500mW to the plus for the 28mm tire. Thatās half a watt.
Now, to really answer the question, we have you know your yaw distribution. Most riders have no idea what their yaw distribution looks like, but Iāll just advise you that as yaw angle goes up the Rule of 105 matters more. At lower yaw angles the difference youāll be seeing is less than the rolling resistance benefit.
One more thing to consider! How much climbing will you be doing? The 28ās are heavier, of course. And the slower you go the more your yaw distribution is going to skew to the right (tending more towards double digits) BUT the less aerodynamics will matter. Of course, rolling resistance is linear wrt speed. So it gets complicated.
If it were me, Iād use the 28ās. I donāt have enough info to give you a cogent suggestion.
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Thank you for the info. As you stated I have no freakin clue what my yaw isā¦ i dont even know what that means. I do absolutely love climbingā¦ but I love going very fast in the flats too. I love both about the same
Inserts. Tubeless. Its my thing for a reason. My Trek Domane had scuff marks on the aluminum box wheels from flatting with tubes. My Enve carbon wheels got replaced when they got road rash.
If you are scared at all, my first question is - are you running tubeless and inserts?
Personally I donāt mind spending money to buy some watts, work on aero gains, and have some fun while I still can. Iām chasing KOMs and some watts and marginal gains. Done nearly all of my own home maintenance for 23 years, and last 4 or 5 years weāve only had 1 car (and wife has work car). We mostly cook food, and donāt waste any food. My wife is a pro at finding discounts and bargains. My philosophy is live a little, enjoy the things you like, donāt worry be happy. 
Im tubelessā¦ and I also enjoy chasing KOMās and love marginal gainsā¦ are you saying i should switch back to 25ās? 
Only if you can blame the equipment and not the engine. Last week
On wide 28c / 32c Vittoria Corsa N.EXT tires. Not the fastest tires and definitely NOT aero optimized 
Thankfully I donāt need to revisit that one, yet, but somebody is always targeting me, so when I do then itās time for faster 25c/26c tires!!!
Laws of physics donāt change with time. At the very least not on the time scales relevant here 
But other aspects (e. g. comfort or rolling resistance) may be more important than aero gains.
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But donāt the inserts add weight and detract from your marginal gains?
Iām not scared at all because Iām not running 32mm wide rims and 25mm tires for aero optimization and I wouldnāt even go there.
I think the counterargument is that it isnāt a loss, but a tradeoff: inserts offer you more protection against flatting. Some inserts also positively affect how tires behave, although that depends on the type of inserts. Some inserts participate in the ācushioningā and are part of the suspension. You could run lower pressures safely, for example.
Others are make your tubeless tires into ārun flatā tires: the air pressure compresses them and they hardly interact with the tire walls. The advantage is that this means they have only a small impact on rolling resistance. The latter are to my knowledge also lighter. Vittoria Airliner weigh about 24ā39 g each, depending on the size.
Have a look at the cross sections of the various Vittoria inserts in the websiteās menu (click on Tire Inserts).
Lol yes I guess in some theoretical sense it might, let me go consult my local aerodynamics expert and get back to you on this important question.
Iām guessing running 28c and 32c on wheels optimized for 26c 
is a bigger crime against marginal gains and all things aero.
But hey, you brought up wheel road rash! Iām just trying to minimize human road rash!
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What inserts do you run? Vittorias?
Naturally!
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That wouldnāt be optimal though, if you follow the 105% rule then optimal tyre width on a 32mm rim would be about 30mm, which likely means running a stated 28mm tyre which will inflate wider than that. Just figuring out the least bad way of writing that sentence makes me realise that manufacturers need to do a much better job of explaining tyre widths!
More broadly what hasnāt been discussed is the root cause of WHY a rim that is 105% wider than the tyre is more aero. My understanding at the time it was first discussed (Iām sure others know much more!) was that it was due to the shape of the tyre cross-section and to how smooth the tyre-rim interface was. I.e. If the tyre is wider than the rim that means itās going to bulge out just above the rim which isnāt smooth, and then itās also going to have quite a round shape which isnāt very aero. Vs a narrower tyre where you get a smoother tyre-rim interface and more of a U shape with straighter tyre walls.
Seems to me that both tyre shape and tyre-rim interface would be affected by tyre/rim developments which have taken place since the 105% rule was first discussed, which might at least mean that the aero penalties for going wider arenāt as significant as they were say 10 years ago. Tubeless is one change - not having a tube could potentially change tyre shape a little. Hookless is another - eliminating the hook could make for a smoother tyre-rim interface and also a less rounded tyre as youāre not pushing the bead in as much. And with both rim widths and tyre sizes increasing itās also possible the 105% just doesnāt scale linearly. E.g. the aero penalty for fitting a 25mm tyre onto a narrow rim optimised for 23mm might be bigger than the penalty for fitting a 30mm tyre onto a wide rim optimised for 28mm.
Are Wider Tyres a total WIN? The Aerodynamics. - YouTube
Take a look at this video. Takeaway is: run the fastest tyre you can get your hands on. GP5k was 19 watts faster than Vittoria Corsa Next. 32 vs 28 for the GP5k was within a couple of watts on a 32.6mm wide wheelset. So wheter you run 25 or 28s does not matter as much as running the fastest tyres.
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25mm is Cannondaleās spec for the Knot 64 rim pictured. They think that is the fastest combo. You can argue any other details with them!
My advice to the OP is to uses those 28mm tires and not worry about the extra 2 watts unless heās sprinting at the elite level or riding a TT championship.
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Ah, sorry, had missed that context and agree there is no way I would be running 25mm on a rim that wide! I race on 25mm but thatās on rims with 28mm external width. Seems a weird spec decision by Cannondale, maybe they got a great price bulk buying 25mm tyres since nobody else seems to want them these daysā¦
even weirder is 35mm wide front wheel on the Roval Rapide CLX, and its aero optimized at 25c / 26c! I get a lot of comments about that front wheel when running aero optimized tires.
this.
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You do wonder when a major manufacturer specs that tire/rim combo on their Superbike and says that is absolutely the fastest setup. Specialized specs a 26mm tire on their 31mm wide Roval. (Edit - wow, the front is 34mm!?!)
I would characterize the difference in rolling resistance between a 25mm & a 28mm tire as āsmallā.
The difference in rolling resistance between a tire with an insert and the same tire without an insert is āmaterialā. Weāre starting to talk about integer numbers of watts. So there is that to consider as well.
