Optimal tire width for 32mm OD road rim

Need new tires for my 64mm deep, 21mm inner width, 32mm outer width rims…
With wider being all the rage these days, what is the optimal width should be looking at? 25 or 28?

Based on the following info from bicycle rolling resistance if you use their rule of 72%, for a 21mm inner width rim you should be running 29mm measured, which is likely a 28mm nominal tire. Or am I misunderstanding this?

Currently I have 25mm tires on the rims and they blow up to a 27mm measured with calipers

Optimal for what? Aero? Comfort? Smooth roads? Rough roads?

The fastest on a smooth road is probably going to be a 25 or 26mm tire. But do you want to roll around on rocks? I run 25mm GP5000s on my bike (widest tire I can run) and the lowest I can reasonably go is 80psi.

On rough roads, 32mm tires at a lower psi are showing to be just as fast as skinny tires.

I just bought the same spec rim (Hollowgram R41 ) and decided to run 30mm GP5000s. I’m willing to trade away a few aero watts. Still, I think I probably should have went straight to 32mm tires as the roads around here are rough.

25 mm tires are on their way out, even 28 mm is the smallest size most people run. The trend goes to 30–32 mm. I wouldn’t bother with 25 mm tires, unless clearance (frame or, if you are on rim brakes, calipers) are preventing me from running anything wider).

This is the only “negative” of my first-gen Strada, it only supports up to 30 mm wide tires (width-as-measured). Back when it was released, this was considered “super wide”, now not so much.

i have a systemsix. when looking with calipers the clearance issue will occur first with the lower chain stays and 30mm will be TIGHT based on measurements

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I’m running 28mm GP 5000 S TR on my systemsix with the HollowGram 64 wheels and it’s fantastic.