LOL; I predict too that in 10 to 15 years a new tire size will come out for road bikes so the industry can make more money selling new rims and tires.

It would be a lot easier if the industry would stop using 27" or 26" to identify tires and rims and use the ERTRO designations 630, 622, 590, 584 etc., which are based on the bead seat diameter of the tire.

The strangest designation of tire size is the 27 1/2" for 650b ((584mm) mountain bike tires and rims.

The Autumn issue of Bicycle Quarterly has a brief history of tire sizes and how they came to be named.

I just opened the thread to see how in the world this question could have 27 replies

You can thank Bibendum and these guys

Should add, one can still buy 27 inch TUBULAR's aka sew-ups. But good luck asking for them from a LBS.

LOL. The arguments people get into around here are sometimes pretty amazing.

I thought I had read somewhere that there never were 27" tubular tires/rims, just 700c rims and tires labeled as 27" for the American market, so I just did a quick search, and here is one reference: Sheldon Brown's Bicycle Glossary Tp--Tz

This explains everything! No doubt these are the same guys who failed to warn the rest of the world of the problems caused by not sticking with their French Metric standards for road bikes.

As for the OP's tube-size dilemma, any 700c or 27" tube would be ok diameter-wise, and there are still plenty of narrow-enough tubes out there with Shraeder-valve stems on them.
I put just enough air in a tube to give produce it's basic shape, and if it fits within the tire and not sticking out beyond the beaded edges of the tire, the width is not too big.
And I refer to use the fattest tube that will fit, regardless of the tire's designated width, because a tube that doesn't have to stretch to fill a given width of tire will not lose air as fast when punctured. I can sometimes even ride home after a ride with such a generously-wide leaking tube that doesn't leak too fast, then fix it at home!

Tubs in a 27 would be a real pain to find not only at an LBS but even on line, the only one I could find was a Conti Giro on sale at Amazon for $28.

As Darwin says, "27 inch tubulars" was a term erroneously used by manufacturers (like Schwinn) who should have known better. What they called 27" tubulars are the same 622mm diameter sew-up tires that have been used for decades.

Tubular Tires|Sheldon Brown

This 1973 Schwinn Paramount catalog page was probably written by some marketing person who wouldn't know a sew-up tubular tire if he saw it.

This is true, not sure why they did that with the sizes other than perhaps try to end any confusion when people in the US and England shopped for tires they didn't get confused about the various sizes. Which I think is a stupid reason, sort like saying people are too dumb to understand different size markings! But regardless I remember the days of buying 27" tubulars because that's how they were marked for sale.

Over here we speak Dutch and had a lot of 27" (630) tires back in the day. In the eighties the switch was made to 700c (622)
635 is also very common on certain types of Dutch bikes.

Its done to mess with our heads. Goofy as the ATB scene and the 29er. The 700c is basically the same as 29er's but they had to make that subtle difference. For another I have vintage rims AND tires stated for 28" clincher. It is weird though and might be Michelins oddity but the 28" rubber fits loose on the 28" Weinmann rim, yet it all works...lol.

Many Japanese 80's bikes listed 27" tubulars. Then you have the Continental brand doing the same with 27 - 3/4 tubular.
Or this:
Vintage Used 27 inch Wheel Sew Up Tubular for Road Bike Ukai Rim Sunshine Hub | eBay

Yep; as Sheldon says, "This careless nomenclature still causes confusion, and people often imagine that there is a different '27 inch' size in tubulars as there is in clinchers."

I agree on ETRTO/ISO, and perhaps one of my New Year's resolutions will be to switch completely to its use.

The "27.5" nomenclature isn't so weird in context -- the MTB industry likes coming up with their own jargon for everything, and after years of "26" and "29er" wheels, the 650B (oops, ISO 584) standard gives them a middle ground, something new to sell whether or not it solves any problems. And of course, they needed a special term for it, so they took the average of the other two nominal wheel sizes.

Interesting bit of history, I didn't realize the Dutch used 27" tires.

i don't like 27" wheels, 'cause you can't get a tire more narrow than 1" (25mm), and 1" paselas are usually more expensive and harder to find than 23c.

(my riding conditions don't require paselas wider than 23c.)

and c&v bikes seem like they're more marketable with a 700c wheelset.

You have to be really careful with 23mm 700cs in this town- I got tired of flats all the timet and went to 25s- then to 28s. I found that I could ride faster on the bigger carcass. So I don't find that the 27" size has any disadvantage.

Rolling resistance wise there is none, but there is something called inertia which is how a bike will hold it's line, and affects handling and that's where the differences lie. Here is a copy of part of the article in Bicycle Quarterly that speaks about inertia and how it affects handling:

"Handling Why do larger wheels feel different? The front wheel’s rotational inertia affects how a bike holds its line, both on straightaways and in corners. Larger wheels and/or heavier tires/rims have more rotational inertia, making it harder to turn the front wheel to initiate cornering. If your front wheel has too much rotational inertia, it becomes difficult to change your line in mid-corner, for example, to avoid a pothole or to round
a curve with decreasing radius. With too little rotational inertia, your bike requires constant corrections to stay on course. You want a wheel/tire combination that is just right, with neither too much nor too little stability.

Another Bicycle Quarterly test had three people ride three bikes with identical geometries (fit, trail, bottom bracket height, etc.), but with different wheel sizes (26 inch, 650B, 700C). All test riders independently
found that they preferred smaller wheels for wider and heavier tires, and larger wheels for narrower, lighter tires. When we calculated the rotational inertia of the wheels, we found that these preferences all yielded similar values. Based on that test, we concluded that 700C wheels are best for narrow tires up to about 30 mm. For wider tires (30 to 42 millimeters), our testers preferred the somewhat smaller 650B wheels. Tires wider than 42 millimeters handled best on even smaller 26-inch wheels.

It is no coincidence that the outer diameter of bicycle wheels has remained relatively constant, somewhere between 26 and 27 inches, since chain-driven bicycles were first were developed 130 years ago, despite much experimentation with other sizes. Motorcyclists have arrived at similar conclusions; they went to smaller wheels when their tires became wider and heavier, so that the rotational inertia remained the same.

This means that you should first decide how wide you want your tires to be. The wheel size will follow from that. If you love narrow tires, you should use a larger wheel. If you prefer wide tires, your wheels should be smaller."

End of copy from Bicycle Quarterly, I hope that helps; if you want to read the entire article you can download this PDF file; see:
WHEEL SIZE MATTERS - Adventure Cycling Association

The increasing popularity of the 700c wheel size for very wide mountain bike tires ('29er') seems to indicate that this preference is far from universal.

True, not sure why that is when the facts are the facts.

Maybe it's not a good idea to conclude universal "facts" about people's preferences based on what three individuals indicate they like.

But it would sure help polling companies, market research, medical trials, etc. if such small sample sizes were adequate.

Koga-Miyata had a few 27"-wheeled touring models in the seventies, but I suspect those frames were shared with the US market. However, Raleigh was a very popular brand in Holland, which would explain the bike shops stocking that tire size.

A few individuals is not correct, this has been a know fact for many years in the motorcycle world and the bicycle world.

I'm not convinced there is one ideal tire size, but having said that, I also can't believe that marketing and production costs have not weighed into the sizes we have. They can't be here purely from well-informed preferences.