If your wheels are designed for 28s, I'd TT with a 25 in front for improved aerodynamics, and stick with the 28 rear so you get the improved flat protection, comfort and reduced rolling resistance. On most TT bikes the rear wheel is drafting in the seat tube cutout, and is also in dirty air already, so the aero gains 25 vs 28 are minimal (and ~0 if you are running a rear disc).

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ECMGN Therapy Silicon Valley:
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Could you elaborate, how to determine if the wheel is good to run 28mm? I have set of Ron wheels on my TT bike and DT Swiss Arc1400, the 28mms tyres fit well, but I am not sure if it is the right choice to win on the rolling resistance.

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Oh look, this thread again...

• My road bike has a 28/28 Conti GP 5000 S TR setup on DT Swiss Arc 1400 with 20mm internal width. There's a small aero penalty with a small comfort gain.
• My tri bike has a 25/28 Conti GP 5000 TT setup on DT Swiss Arc 1400 with 20mm internal width. This is to prioritise aero over anything else, but I appreciate the difference is marginal.

Just search hookless and you'll find 150 pages of debate about tire size and pressure.

-Narrower frontal area is more aero, all else being equal (a HUGE assumption that all else is equal. Rim shape, downtube profile, rim width, etc)

-As the road surface becomes more bumpy the wider tires at lower pressure become the lower RR option.

-RR increases linearly with speed, aero drag increases exponentially.


There's 1000 different scenarios even just for one singular rider. A high speed TT on smooth roads with narrow rims is probably 25's, maybe 23's. Strade Bianchi where you're sheltered in a pack all day and riding on bumpy gravel in key sections is surely 28's or bigger.

Any wheel can run a 28mm tire.

Assuming it's a deep rim, the issue is if it was designed around 28mm tires from an aero perspective. For example, I have some HED J6 wheels, and they were designed around 23mm tires (you can run 25mm with a small aero penalty, and 28mm with a bigger one).

Looking at the RON V6 wheel, it's an 18mm inner rim width (25 outer), and will be faster with a 23mm tire than a 25.

For the rear wheel the combination if it being less important aerodynamically and bearing more weight (so more rolling resistance) than the front tips the advantage towards using a 28.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I was trialling the new Ron Ultron rear disc last night in a race. A 28mm tyre comes up as 30mm in that wheel. That’s getting big. I fully take the point about dirty air but most current seat tubes are not wide enough to provide an effective leading edge to that. Damn comfy though.