Most computers offer the option of entering the tire rollout distance, which you can measure.
Or, just get a gps computer.

I have read this entire 15 year-long thread, and I don't think I understand anything better than I did when I searched up the same question as the OP.
No fault of the posters, though people tend to add bits of extremely minute partial detail in dribs and drabs, probably based on their particular experience.
I think really you'd need a well written and edited book with diagrams to properly explain the history of and current naming conventions for tires and wheel sizes.
In any case, speaking of personal experience, I just installed a 45mm winter tire marked 28" on a rim marked 29, which I bought because I was told it was the same as a 700.
It fits fine! All perfectly rational :thumb:

The link given by the first responder in this thread should explain it all to you. If you read that and still don't understand, then make a new thread and ask your specific questions. Otherwise others might address the other stuff written and it'll get confusing trying to figure out what was meant for whom.

Blame it somewhat on the mfrs. trying to wow buyers with "new and better" sized tires and old standards. There is the ISO (International Standards Org) which is the most rational and accurate measuring system and printed on almost all current tire sidewalls along with the non-sensical sizes we're most used to. 700c/28"/29" all have a BSD (Bead Seat Diameter) of 622mm which is what you should see on your 28" marked on the tire. ISO widths will also be marked in mm so should read "45-622", easy. As mentioned, post #1 gives a link to the Sheldon Brown article that explains it all when you want to know sizes for other tires and eye opening when buying 26" tires. Hopefully this answers the questions, and this thread can rest in peace.

I've wondered if it should be blamed on the mountain bikers. At least for the manufacturer's continuing this practice. Reading the mountain bike magazines BITD, it seems they talked in terms of outer diameter for tire size and which would be better suited to what terrain.

Not that I'm against mountain biking. So maybe It's the manufacturer's ad people getting the mistaken belief from mountain biker lingo that they wished to continue the practice.

Clear as mud.

ISO 5775 (international standard for labeling the size of bicycle tires and rims) kinda throws a little more mud on the subject.

https://en.wikipedia.org/wiki/ISO_5775

(my apologies if this has been mentioned previously)
reporting from Chiang Mai, Thailand

It's really simple - 28x1-5/8x1-1/2" is the same size as 29" and 700C - they're all 622 mm on the bead seat.
Meanwhile 27" is larger than all of them, at 630 mm, except British 28x1-1/2" aka 700B which is 635 mm.
This is why the ISO standard (formerly ETRTO) is the only way to be sure that you get what you need: 25-622 is a road race tyre and 70-622 is a 29er tyre.
27-5 and 29er are both marketing terms for MTB wheels, 27-5 is actually the French size 650B while 29er is 700C so they're not new sizes just confusing names.
The British 28" was for traditional rod brake roadsters which are still common in India, the more common "English roadster" size is 26x1-3/8" which, it should be noted, is not the same as 26x1-1/4" or 26x1.25" or 26x1-1/2" or 26x1" or a different 26x1" ... just use the ISO size and avoid all this confusion.

700c is an obsolete French designation, IIRC...
27" and 29" are badly named American marketing names related to Overall tire diameter, when mounted and inflated... and 28" is a bad English marketing name, along those same lines....

650b is a french thing.... again.... i think.... maybe.... kinda-sorta.

and the 700 or 650 specs simply don't exist, but are approximations of overall TIRE OUTSIDE Diameters... as are any INCH specs shown regarding RIM Sizes. 20" rims are actually about 15 3/4"across the diameter...., etc.

there are 12", 16", 18", 20", 22", 24", 26", 27", 28", and 29" bike tires being sold... with variations of Actual bead Diameters on several of those tires...

Continental and Schwalbe seem to being creating their own bead specs at times... the darn things refuse to bead up on the rims they were SUPPOSED to fit on... IRC made some of those too... :notamused:

And then SCHWINN decided to create their VERY Own, Proprietary Tire Bead sizes and designs...... :foo: wanna buy some 22" Schwinn ROAD tires? oh wait... i tossed all but a few of those turkeys. People were attempting to sell them on 'Bay for less than shipping cost, with free shipping! $5 each.. shipped. :roflmao2:
it was cheaper than the dump costs. Luckily, my local dump allows bike tires to be disposed of if they're in plastic trash bags, at the normal rates per load size....

i hope this adds to the confusions created by way too many stubborn folks bent on clinging to ridiculous specs that never really meant what they were supposed to anyway.. it seems that confusion was the only goal, at times.

did you know that 20 x 1.375" and 20 x 1 3/8" mean different things, in the bike tire world? they do.... fractional and decimal sizes denote a BEAD SEAT DESIGN and DIAMETER DIFFERENCE.... most of the time... but not always... :twitchy:

An ETRTO number shows Actual BEAD Seat DIAMETER, which is a MUCH Better spec to rely on.
i'm sure some hate ETRTO too... especially the stubborn folks.

(i still call "tubular" tires SEW UPS.. :D)

Although I detest responding to a very ancient thread, you’ve got things a bit mixed up. 27” wheels have a 630mm bead diameter. With a “proper” sized tire, the overall tire diameter is 27”. Given that a 630mm bead set diameter is 24.8”, that means the tire should be 1.1” in diameter to make the wheel/tire system 27”. 29” is a designation that mountain bike guys came up with for a 700C (622mm) rim with a 2.25” tire on it.

The French system maintained the same outside diameter of the tire but changed the bead diameter (and rim diameter) to do so. A 700A used a 642mm bead diameter with a 29mm tire diameter to achieve 700mm outside diameter. A 700B used a 635mm bead diameter and a 32.5mm tire to achieve a 700mm outside diameter. A 700C used a 622mm bead diameter and a 39mm tire while a 700D used a 583mm bead diameter and a 58.5mm tire. It was a throughly confusing system and, thankfully, only the 700C remains. Note that in this system a 700C is almost the widest tire that was available under the French system.

The 650 system did the same but with a correspondingly smaller overall diameter. The 650B and 650C still exist as a rim diameter but not the tire system.

Yes, ETRTO is much better, although they get 37mm wide tire wrong since both 35 and 37 can be marked as “35mm” or “37mm”. It has to do with the height and width measurement. However, if you don’t know the old system, heaven help you with trying to find tires for some wheel. And heaven help you if someone comes looking for a 27” tire. It could be an old 27” or it could be the misnamed 27.5.

I guess I have it pretty good here - Up to 5 vehicle tires a day FREE at our dump. Keeps them off the street which is more expensive in labor to clean up. They tried charging $5/tire but then they were getting dumped on the street.

Thanks for the additional information...

and I wasn't the mix-master. :)

they end up in big heaps out in the woods around here.... i think it's $7 each, at this dump... twice that, if still mounted on a rim. YIKES! the dump guys hate doing car/truck tires.
the tire shops take them in, in bulk... to some place that shreds them.

an aluminum car wheel sells for 50 cents/lb, or more, as scrap. cast and forge-able alu... clean.
figure 10 to 12 bucks, each!

Like I said, mud. Mississippi Mud.

It's very simple:
28 is same as 29
27 is bigger than 27.5, 28 or 29

Like I said, the ISO standard is the only way to be sure. Forget about the customary names, those five or six digits are all you need. Never be that guy who goes into a shop to ask for "a 26 inch tyre".

This should clear it up

https://cimg4.ibsrv.net/gimg/bikefor...ec6d97ffcd.jpg

Tube size to use for a bike is more flexible than tire size to use on bike is more flexible than BSD (bead seat diameter of rim of bike). The BSD is fixed. The tire can vary somewhat in cross-section. The tube can vary somewhat in cross-section and diameter.

That reminds me that I need to order a spare 17" (ISO369) for my Alex Moulton.

Fun fact: the CarryMe folding bike uses 8" pneumatic tires.