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I tried to do this research myself, but came up empty.
i'm going to Europe this summer and will be riding some big climbs. I have 23 mm rims and need new tires. Currently riding 23mm tires, but have heard that 25 has lower RR and will be faster. Is 25 the "sweet spot" or should I consider 28.??
25's are perfect. The only reason to ride 28's is for lower pressure (60-70#) for cobbles or maybe rough gravel/dirt roads.
28's have a lower rolling resistance if you choose the right model.
Most frames won't fit them.
Big tyres make your gearing higher.
I just put 28s on my caad10 w/ jet + rims. It's like butter at 60psi. I love it. It's noticeably smoother than 25s at 80.
There seem to be some tire gurus on here that would know better, but from my experience there IS a point at which rolling resistance increases with tire size. It might be a practical limit (larger tires are made less supple because they wouldn't be puncture resistant enough, or the casing wouldn't be strong enough for similar pressures), or a market limit (people who buy city/touring tires care about other things more than the ultimate in rolling resistance, so manufacturers don't make them), but the rolling resistance tests I've seen of 38mm tires are all significantly greater (double or more) than good road tires. I'm not sure what happens at 28mm, but somewhere between 28 and 38 things change.
I like to test ideas at the extreme. For example, I have a set of 42mm Continental Ultra Light folding tires on my commuter. It's a really fast rolling tire as the tread has worn down, and it's now a similar thickness in the tread as a new road tire. At low speeds, I wouldn't be surprised if it's similar to a decent 25 mm tire. Problem is, such a wide, thin tire picks up puncture flats like CRAZY (I know, time to replace them (but they roll so nice)). The wide tire doesn't eject debris as well, and I think part of it is just the odds of picking something up increase with a wider contact patch. Point being, you probably COULD make an extremely low resistance huge tire, but to achieve the necessary properties, you'd have to give up a ton of puncture resistance (at least with today's tire materials).
Are your rims 23mm external? That would be 17-18mm internal. For that rim I would do a 25.
I ride a bunch of sizes. 23mm corsa speeds, 25mm corsa g+, 28mm conti gp4000 ii, and 47mm horizon 640b tires.
My favorites for all around use are the 28mm contis on zipp 30 course disc wheels with 21mm internal width. Fast and comfortable.
For narrower rims on a traditional road bike I think the specialized sworks turbos in 26mm are a great option.