I put these new Panaracer 650x28c from Bell's Bike Shop for a granddaughter on Wheel Master Mach 1 RJ Project 650cc Wheelset HG Road, which advertises a 15mm internal rim, which I verified. As you see here at 70 psi they were only 24mm tall and 25mm wide. I assumed they were fakes or mis-labeled. After several emails Bell's "Service Team" finally replied and I quote "Listed tire width vs actual measurement with calipers will often not match and will vary from brand to brand. Rim width, tire pressure, and manufacturer molds will all cause variances in the real world measured width on the tire vs the advertised size. just took calipers to several bikes and the shop and the listed width vs actual measurement varied between 3-6mm across several brands." In my 75 years and dozens of road bikes for my wife, kids and grandkids bikes I have never seen anything like this, have I just been lucky.

Looks like you're buying from a retailer so they are not going to know the intimate details of the tire's fabrication. Yeah, there are variations. The manufacturer/designer might base their advertised measurements on a different size or brand of rim than you have. If the tires fit, if they ride with no issues, I'd just keep them and not worry about a 3mm discrepancy.

That's an odd pressure to be running them at. Recommended is 105:
> Recommended Air Pressure: 105 psi
https://tokyobike.co.uk/products/pan...71-cs650-sport

Kind of an ancient hold back from before people knew wider tires with lower pressure were faster and more comfortable. Personally, 15mm internal rim width is kind of rare now too.

Just for laughs I looked at the Rolling Resistance site. They tested a pasela, different model than yours. The tire was labeled as 38, they measured it as 35

When you're dealing with air filled rubber things I think dial calipers may be overkill.

The bike is light and she is small for 8 years old at 50 pounds, the Silca Inflation site recommend 74 front and 76 back for a 75 pound rider/bike in our Airpark with excellent roads.

IME, Panaracers have been measuring on the narrow side recently. Mounting them on narrower rims, such as 15mm interior, keeps the profile narrower. When I mount the same tire on a rim with 18mm interior, the width is close to what is stated. I have never checked tire heights.
I have a set ov Vittoria Zaffiro Pro Control labeled as 30mm. But they measure a bit over 32mm on 18mm interior wheels.
I check widths only for clearance issues, not to check tire specs. I generally use the widest that will fit with at least 2mm of clearance at all points of possible contact.

the nominal width on a tire is kind of meaningless unless the rim internal width is factored in. I run 25mm Vittoria Rubino Pros on two wheel sets - on the 17mm width wheels, these « 25s » are an actual >27mm diam. On the older 15mm width wheelset, the tires are actually 25mm diam. Likely your tires were « calibrated » for something wider than a 15mm rim. 15mm is effectively obsolete these days.

It’s twu! It’s twu!

"Coming, and going and coming, and going".....

Assuming Panaracer took the lead from the rewrite of ETRTO tire and rim compatibility standards a few years back, they will be labeling nominal tire widths based on the wider rims now typically in use. Since you put the tire on a narrow rim, you should expect it to have a narrower profile especially at that pressure.

Otto

True in my experience.

Google says “modern standard” rim internal width is 19-23mm. I expect the manufacturer is giving width based on that.

So yes, it’s probably true.

Coming and going and going and coming, and always too soon.

Epic fail to recommend such high pressure for a 28mm tire for a person and their weight you don't know.

Using an online calculator is sound advice.

Car tires can be dramatically different from one brand to another with the same numbers. Same with motorcycles and, obviously, bicycles. Some "run small" and others "run large" compared to the listed size.

And don't get me started on bike clothes.

I forget where I copied this from:

Ooops. I was having a hard time seeing where the rim is.
I just had the same problem with Schwalbe 700 x 35/37 E marathon tires. They were 33 mm on my Dyad rims. Instantly ditched, I'm not riding these rocks.
My SMP tires were 34.5 and still less than my desired 36 mm. So I got 38/ 40s, actual 37.

It would certainly affect measured size, though. Epic fail not to suggest reasons for the measurement to be off, which is what the OP was asking.

Or any clothes for that matter. Or shoes.

The last time I was in Italy riding (2013) I was easily an XL American. Went to a high end shop and saw a really nice kit I wanted. The 3XL jersey was too small. Didn’t bother trying on the bibs. A guy in our group was a slight M American, An XL jersey he wanted was too small.

So in your 75 years, you've measured every new tire the same way and have never seen this? Or might this be one of the few tires you've even really measured. And you are doing it wrong as far as measuring height. The BSD of a tire is measured to the shelf the bead sits on, not the outer top edge of the rim.

The Mavic's I just measured have about 5mm from the edge to the bead seat. So there probably is enough distance to make that tire come out as being 28mm tall. Tire size is only width and BSD. Though it is generally acceptable that the width of a bicycle tire height will add the same amount of distance to the BSD so it's overall diameter and circumference can be figured. But it happens on both sides of the rim, so multiply by 2.

Still are you certain the tire is seated evenly all the way around the rim. I've never used that brand, but many of the other's I use the witness line sits further away from the edge of the rim.

And despite any of this, I fail to understand why 2 or 3 mm matters for a bicycle tire size. It won't make that much noticeable difference to my ride or performance. And it won't be on my bike that long anyway before it's time for another.