Sy Reene

Random thought.. is a tire's width-as-measured best defined as ideally the inside carcass width, vs. just taking a caliper and measuring the inflated tire? Reason I ask is for example, some tires have the thicker tread (that runs down the center) extend and wrap further around the tire toward or all the way(?) to the bead. Some tires only have the thick center tread on the tops of the tire and a caliper wouldn't be touching this section when doing a measurement.

If the heavier tread thickness could be eg. 1mm thicker than sidewall thickness, and doubling that when measuring a tire's inflated width, could be inadvertently adding comparatively 2mm to one tire vs. another. Maybe an example is a Gatorskin vs a GP5K ? Both have the exact same 26mm measured width according to BRR's site, but is one including a bunch of protective rubber, while the other is not. From an inflation/PSI perspective, is the GP5K more air than a Gatorskin. The default conclusion being that the Gatorskin should be inflated to a higher PSI ?

Iride01 

I've always felt that tire width is measured by the manufacturer as the width of the tire casing. Which isn't much difference internal or external (to the casing). The compounds added outside the casings add to the width and height, but are not counted in the ISO / ETRTO size.

I think I got this idea from one manufacturer's website long ago, but certainly it might only pertain to that manufacturer. Don't remember which site it was.

Now if you are talking about the width specified in the big flashy letters on the sidewall and box, then I never pay attention to them.

mstateglfr 

Why wouldnt 'width as measured' be the width of the tire as measured on a rim? Put a tire on a rim, pump it up, and measure. There- width as measured. People care about the width of a tire on a rim because they have to figure out if it fits between their chainstays and fork legs.
The casing thickness is beside the point. If a tire is 28mm wide inside the casing, but has a 6mm casing, then its actually a 40mm wide tire for all intents and purposes when it comes to fitting the tire on your bike. The 40mm measurement is whats important for fit.
And obviously rim measurements and PSI need to be standardized for this to mean much of anything to anyone.

...maybe I misunderstand the question though.

WhyFi

They're measured as mounted.

As for your example, I wouldn't worry about it. a) big whoop - you're going to take inflation recommendations with a grain of salt, anyway, given preference, road conditions, gauge inaccuracies, etc b) in your scenario, the extra tread thickness would also result in a less supple casing, probably making up for the small psi recommendation difference and then some.

WhyFi

He's referring to WAM (width as measured) and RAM (radius as measured), figures that have recently been pushed forward by a number of manufacturers, primarily those making tire and/or frames, so that end users have a better idea of what to expect by way of tire clearance when using various combinations of rim and tire.

Sy Reene

Yeah, I already do what I do, and maybe not a big deal. However to note, a 2mm width difference can often change the PSI recommendation on some calculators by over 10psi. Makes you wonder if something like what's being discussed here could be part of the reason for frequent disconnects on sensible tire pressures?

WhyFi

And, again - think about the increased stiffness of the casing, the inaccuracy of most pump gauges and then the percentage of the overall deviation (I assume that this 10psi difference is on skinny tires are the pressures are probably in the neighborhood of 100 or more).

No, doesn't make me wonder that at all - I wouldn't place any blame at the feet of tread thickness variation, within a given WAM, as a factor for the disconnect. Even among forum members, the majority of whom would classify themselves as enthusiasts, WAM is a relatively unknown concept and the inflation calculators that use them are pretty new-school. Couple that with the anything goes over-sized tire casings that are still available on shelves, the newer, more conservative tire sizes, the wide variations in rim widths on the market, the factors listed previously and the old-school habits and "knowledge," etc. etc - it more than adequately explains the differences in opinion.

chaadster