robertj298 

I've had both 700 and 27 inch tires that differ vastly to what
their listed widths are.

John E

Same here. In my experience, Continentals run small (e.g. 25-26mm on a 700Cx28 callout) and Specialized run large (e.g. almost 30mm on the same callout size). I can put Conti 700Cx28 on the Bianchi, but not Specialized.

Even more frustrating is bead diameter vs. rim diameter. There is no way I can get a 700Cx28 Michelin folder on a Campag. Omega rim. Contis are a tight fit, but they do work.

VtwinVince

Yup, drives me nuts. I have two sets of Schwalbes which are so small I can't mount them.

Wildwood 

Ever manufacture an 'inexpensive' commodity product? Overseas?

edit: if it passes internal Quality Processes, then you (as manufacturing) could care less about the competitor's product. who knows what the design people are targeting? Stacking all the equipment tolerances, I try not to buy tires too close to my fork/stays allowance.

nlerner

Because s**t happens.

SurferRosa

Are all rims, tubes, and air pressures the same?

pcb 

Sorry if this is long and tedious, but what the heck else is there to do?

In terms of tire width, I think there are 3 main reasons for inconsistency:

1. Lack of Standards: Actual tire width is highly dependent on rim width---actual tire width increases on wider rims. So the marked width number is semi-meaningless without knowing the rim width used to calculate that measurement. But there doesn't seem to have been any industry standard on what rim width to use to measure/state the tire width. that leads to lots of confusion with the wide range of rim widths in the market today. Is a 700x28 Continental GP5000 tire 28mm on a 1986 Mavic Open Pro rim with 15mm inside width, or a current Mavic Open Pro, with 19mm inside width? What happens on a HED Belgium+ rim at 21mm internal width? The tire mfrs don't tell us, so we have no way of knowing except through trial-and-error, or the interwebs.

With the introduction of tubeless tire/wheel systems, and tire/rim sizing/fit getting really critical, there is a new standards scheme brewing. Lotsa info out there on the web, I tried reading some and got a headache. I once buttonholed a Panaracer rep at a show and suggested tire sizes should be listed as diameter-tire width-rim width, so we'd know. In my perfect world, a 700x28 tire that was 28mm on a 19mm internal-width rim would be labeled 622-28-19. Then we'd know how much narrower/wider it might be on a 17mm/21mm rim. [sarcasm]I'm sure that suggestion went right to the top.[/sarcasm]

But what happens when the labeled width is "wrong?" That leads to reason #2 :

2. Manufacturing Inconsistencies: Tires are made with molds, and for industrial reasons I don't at all understand, it's evidently not unusual for a tire to pop out of a mold wider or narrower than intended. They're not going to throw out the mold when that happens, and the size/width labeling is molded into the rubber, so the tire will remain that "inaccurate" size until the mold wears out. "Inacccurate" in quotes, because, you know, the tire will likely be the labeled width on some rims----we just don't know which.

And when the mold is replaced, it will likely be replaced by an identical mold, with the identical tire width popping out.

Examples of this are legion. The Conti GP4000SII 28mm tire was famous for plumping up to 32mm on wider rims, so much so that when Conti introduced the 5k, they added a 32mm size, and made the 28mm narrower.

I suspect the GP4k/28 was a surprise when it popped out of the mold, and Conti never bothered to change it. I remember the original Grand Bois Cypres was labeled 700x29, but was usually 32mm+ on common rims at the time. To Grand Bois/Panaracer's credit, they did eventually change the molded width to 32.

3. Marketing? I'm not fer sure about this, but when suddenly wider tires started to get popular, there was a loooong string of years where Panaracer Pasela tires were ridiculously labeled undersized, on the common rims of the day. 700x32 tires were like 27mm. My memory's hazy on this now, didn't take me long to stop using them 20yrs ago, but it seemed pretty consistent across the line, every Pasela was 5mm narrower in real life---so it felt more like a marketing exercise than production errors. The Paselas were eventually changed, and when folks were selling them there were always questions about which label they had, the old or new. Because the old-label tires were stupid narrow.

I kinda feel like there were other examples, from other mfrs as well. I maybe kinda remember there were tires with one width molded in, but a wider width shown on the hot-patch label. But I'll leave it with a ??? since who knows? And this far removed in time, it doesn't matter much, except if one of those tires is in a vintage frame and you're trying to figure out what actual width might fit. Hint: never trust the label, always measure the actual width.

Tire and rim actual diameter matter for mounting, and that's maybe a whole other topic. Likely similar issues there.

Tire diameter must also be dependent on the tire molds. There's a known, definite standard for diameter, but I don't think there's any standards body checking/enforcing that. Likely up to mfrs' QC staff?

I remember a run of cheapo tires on new Fujis/Panasonics in the early '80s that were fine up to the labeled 75lbs recommended max pressure. One pound over that and they'd blow right off, regardless of rim. I think tires/rims are supposed to be able to got to twice the highest recommended pressure before blowing off, but again, who enforces that? Fuji replaced all the tires for us, Panasonic didn't.

And rims are just long extrusions rolled and welded/pinned to make a hoop, like a neato Play-Doh Fun Factory toy. If the Play-Doh rim extruder is off by a few fractions of a mm, you've got a nightmare rim that is either holy heck to mount, or lets the tire blow at low pressure. I think the blow-offs get culled by QC, but the nightmare-tight ones often get shipped. I watched some rims being extruded, I mentioned that folks were complaining about tight tire fits, and was told basically: "Yeah, we were rolling them a little too long, but we took care of that." The "a little too long" ones shipped on hundreds of thousands of bikes.

What it comes down to for me, an old guy raging against aging, who wants to cram in as wide a tire as possible on everything, is I have to have a sheet-ton of tires around, and it helps immensely to get familiar with which tires are accurate on narrow rims, and which on wide rims. And I have to hope for consistency.

I use a lot of Soma Supple Vitesse tires, with the very nice high-zoot Panaracer supple casings, and cheaper than Compass/Rene Herse. They are what you might call undersized, meaning they seem to hit their labeled width on wider rims. So a 33mm Soma SV will be 33mm on a wide-ish HED Belgium/+, so probably range 29-31mm on vintage/narrower rims. Compass/Rene Herse are similar.

Grand Bois tires I've used, 28/32, and newer Vittoria Corsa+ tires in 28/30 are kinda oversized, they're close to labeled width on narrower rims, so they'll plump up wider on wider rims.

I've used some Conti 5k in 28/32, and they seem to be "undersized," accurate on wider rims, narrower than labeled on narrow rims.

I haven't used many Panaracer-labeled tires in a while, lotsa Panaracer-made tires, but with other folks' labels. Not sure what's happening with them width-wise in general these days, though I do know folks happy with some of the slick Gravel Kings on vintage bikes. I did snag a couple pair of uber-light Japan-market Panaracer Competition SX Protite 700x28 tires on the cheap, and they're plump, 30mm on wider rims.

If you're not sweating every last mm, and/or you're not as retentive as stunod me, you can just scratch your head and not worry too much. But if you have a frame that, fer instance, is too tight for comfort with 28mm actual width, and you really want 26mm i/o 25mm, 'cause, ya know, at narrower widths every mm counts, then what you've got is a headache.

What's maybe working for us moving forward are new tire/rim sizing standards. What's working against us is the prevalence of tubeless systems, which need unholy tight-fitting tires/rims to seal in the sealant.

Dfrost 

^^^ Nice write-up, pcb .

FWIW, the Compass and Grand Bois tires I’m using (700x28, 32; 650x38) measure true to width or maybe a mm bigger on 23mm (outside width) tb14’s and Pacenti Brevets, except for your correct assertion about those GB Cypres 700x”30”mm. I’m just about to replace my last of those after almost 4000 miles on the rear (following 3500 in front). The replacement Compass (haven’t bought any as RH yet) Stampede Pass 700x32 EL should be a little bit better fit under the Marinoni’s fender boss under the rear brake bridge.

Bad Lag

One thing very few people understand, even engineers, is that EVERYTHING has variation in its characteristics. Whether it is a dimension (size) or a mass or,... whatever. It is not a fixed value. Typically characteristics are given as "nominal" values (average/mean, median, typical, target, ...).

Even then, the actual tire width can be dependent upon the rims they are on or the pressure inside. How would you like to decide whether to use this tire or that one when the sizes are quoted at two different pressures mounted on two different rims?

pcb 

Jeepers, I totally left inflation pressure out of the width calculation. Now I have to reformulate my perfect world. I want to say that the tire pressure is used is maybe relatively unimportant, as long as it's an appropriate pressure for that tire width, but "appropriate" is inexact, and covers a wide range. So in my perfect world, width is stated at highest recommended tire pressure, which shall be molded into the tire.

So the size spec for a 700x28 tire, that is 28mm at highest recommended pressure (HRP) on a rim with xxmm internal width should read:
622-28-xx-HRP

A 622-28-19-95 size would tell me the tire should be 28mm on a 19mm internal width rim at a pressure of 95psi. That would let you know that it'd be a little narrower at lower pressures, a little wider on wider rims, a little narrower on narrower rims. If I knew my frame is too tight at 28mm actual width, but I'm running older rims w/14mm internal width, I'd know these 28s would be closer to 26mm actual and should fit.

I could envision an argument to standardize the internal rim width for all mfrs, just to save us doing a little less math.

Make it so.

DiabloScott

In order of disparity:
1. Differences between manufacturers
2. Differences on standardized rims used for the nominal measurements
3. Variance in product of the same manufacturer.

I think it's very unlikely that you'd see a substantially different width in a road bike tire between 90 and 120 psi on the same rim; but I haven't checked.

There used to be a story that one manufacturer would make (eg) a 22mm tire and label it as a 23mm tire so that when you compared weights among 23mm tires theirs would be lighter. You don't hear that so much anymore.

And yeah, my bike that can take Vittoria 27s with plenty of room won't fit a Conti 28 at all... I don't know if that's a consistent difference between the two brands, but it's enough to make a different decision next time.

canklecat

Tire design differences too. I like Continental Ultra Sport II -- good and cheap -- and Conti Grand Prix Classics, great affordable skinwalls. They ride very similarly, but are very different in fit.

The Ultra Sport II are notoriously tight fitting. I needed a Kool Stop bead jack to seat them. Whenever I ride with those tires I stuff the bead jack into a jersey pocket in case I get a flat.

On the plus side, Ultra Sport II in 700x25 will clear my Ironman's GPX rear brake bridge, no problem.

The GP Classics (700x25 only) have a slightly better ride (much thinner sidewalls, translucent through the reddish brown skinwalls), but the raised center tread rib would just barely clear and any slight road debris would scrape. After a few rides, including some sessions on the indoor trainer, it scraped less after riding through a wet spot. Now it clears, no problem, and there's plenty of wear remaining.

And no need for a bead jack with the GP Classics. I can mount them with my hands, despite aging joints and arthritis. Very helpful after switching to latex tubes -- less risk of pinching the tube and causing a cut that would be difficult to patch on latex.

Vintage Schwinn

Automobile tires have similar issues with stated printed sizing from manufacturer to manufacturer and also sometimes within tire brand's various models.
Folks often see this when for example they fit larger and wider wheels on to vintage sporty cars. Their goal is to fit larger and wider wheels with low profile wide tread that improves traction and road handling. Often these persons want to completely fill the wheel well for just the right look. Often what happens when people try to push the envelope, they find out that certain brands' tire model offerings clear everything perfectly, and some will rub the inner fenders, even though these tires all have the same exact stated printed sizing on the tire sidewall, meaning that the wheel size, aspect ratio, tread width, and speed rating is the same classification. Most automobile tire manufacturer websites and national tire dealer websites have other comparative details such as inflated circumference in milimeters so people can have a better clue which might provide just enough clearance, more than say another of the same size. The only sure fire way to really know is to generally monitor the automotive forums for that particular marque of car and see what the general consensus might be on the that topic of tire size. Another important consideration is manufacturing tolerances and factory assembly on the vehicle in question. It is a serious factor. For example, your 1965 Mustang convertible, or 1966 Corvair convertible, or 1969 Camaro, or 1973 Porsche 911 might vary as much as 7mm or even 10mm from side to side, that you might not be able to be 100% certain that just because Lucas in Little Rock says that it fits but Bubba in Nashville says that it only fit after he rolled the fender lip outward with the old Boog Powell Louisville Slugger baseball bat. Spend just two or three minutes on any automobile website and you'll see that brand & tire size fittment is always a huge issue when trying to find the widest/largest tire that might fit without modification to the fender lip. I guess there is no wonder why bicycle tires are not any more exactly interchangeable.
One thing is certain though, both bicycle tires and automobile tires are significantly better today in 2020 than they were in 1980.