For my road bike's rear wheel that I ride daily, I use a DT XR331 rim with 25C tires,
or with 23C tires at my own risk and outside manufacturer recommendation
but this is originally an MTB rim.
The XR331 rim is extremely narrow compared to the next wider rim
with a nominal inner width of 20mm,
and lately there are increasingly more road bike rims wider than this
（especially carbon tubeless ready rims）.
The next wider after the XR331 is
the XR361 rim.
This has a nominal inner width of 22.5mm,
but when actually measured it came to about 22.8mm.
With Stan's lightest aluminum MTB rim, the Crest MK3,
the inner width is 23mm both nominally and measured,
and after Crest, the Arch, Sentry, Flow, and Baron
increase by exactly 3mm each.
Of these, only the Crest is narrow enough to accommodate
32C cyclocross tires, but
when fitting a new Kenda CX tubeless tire
at 2 bar of air pressure
the outer tire width comes out to around 34mm or so.
Against cross-country 29-inch MTB rims around the width of XR361 or Crest,
if they're nominally 30C or so,
or claiming to be 32C but obviously looking quite narrow,
the tread shape becomes a stretched tire, but
width-wise alone there shouldn't be a regulation violation—
combinations that stay under 33mm might exist,
so I did a little investigation.
I mounted a new tire on an XR361 rim
tubeless at 2 bar pressure and measured the width.
In actual racing I think they'd run around 1.6 bar or so,
but considering tire manufacturing tolerance I went a bit higher.
More than that, tires getting wider from age and use
seems to be a larger variable factor.
Panaracer Gravel King SK TLC 32C
The Gravel King SK does have a tubeless-incompatible model,
but this is the TLC (tubeless compatible) version.
There's barely any visible difference, but you wouldn't mix them up—
the tubeless-incompatible model only comes in 26C and 28C,
so there's no way you'd buy a 32C and have it be tubeless-incompatible.
↑Nominally 290g
On the XR361 at 2 bar it was about 31.8mm.
For a 32C tire in stretched condition, that's quite narrow.
Vittoria Terreno Dry 31C
From the Terreno series which comes in Dry, Wet, and Mixed versions,
this is the Dry.
↑Nominally 410g
On the XR361 at 2 bar...
32.9mm. Way too close—
if the rim becomes a Crest or the tire widens with age it'll exceed 33mm.
Kenda CX Tubeless (not the X-Guard) 32C
After the fact, but we had a small restock.
Since some sold quickly, I got permission from a customer to
mount it briefly on an XR361 rim.
↑Nominally 380g
I couldn't weigh it without a rubber band since
the rubber band weighs less than 1g.
On the XR361 at 2 bar...
Around 33.4mm. That's out of spec.
The Kenda CX Tubeless tends to exceed 33mm when the rim is wider.
or with 23C tires at my own risk and outside manufacturer recommendation
but this is originally an MTB rim.
The XR331 rim is extremely narrow compared to the next wider rim
with a nominal inner width of 20mm,
and lately there are increasingly more road bike rims wider than this
（especially carbon tubeless ready rims）.
The next wider after the XR331 is
the XR361 rim.
This has a nominal inner width of 22.5mm,
but when actually measured it came to about 22.8mm.
With Stan's lightest aluminum MTB rim, the Crest MK3,
the inner width is 23mm both nominally and measured,
and after Crest, the Arch, Sentry, Flow, and Baron
increase by exactly 3mm each.
Of these, only the Crest is narrow enough to accommodate
32C cyclocross tires, but
when fitting a new Kenda CX tubeless tire
at 2 bar of air pressure
the outer tire width comes out to around 34mm or so.
Against cross-country 29-inch MTB rims around the width of XR361 or Crest,
if they're nominally 30C or so,
or claiming to be 32C but obviously looking quite narrow,
the tread shape becomes a stretched tire, but
width-wise alone there shouldn't be a regulation violation—
combinations that stay under 33mm might exist,
so I did a little investigation.
I mounted a new tire on an XR361 rim
tubeless at 2 bar pressure and measured the width.
In actual racing I think they'd run around 1.6 bar or so,
but considering tire manufacturing tolerance I went a bit higher.
More than that, tires getting wider from age and use
seems to be a larger variable factor.
Panaracer Gravel King SK TLC 32C
The Gravel King SK does have a tubeless-incompatible model,
but this is the TLC (tubeless compatible) version.
There's barely any visible difference, but you wouldn't mix them up—
the tubeless-incompatible model only comes in 26C and 28C,
so there's no way you'd buy a 32C and have it be tubeless-incompatible.
↑Nominally 290g
On the XR361 at 2 bar it was about 31.8mm.
For a 32C tire in stretched condition, that's quite narrow.
Vittoria Terreno Dry 31C
From the Terreno series which comes in Dry, Wet, and Mixed versions,
this is the Dry.
↑Nominally 410g
On the XR361 at 2 bar...
32.9mm. Way too close—
if the rim becomes a Crest or the tire widens with age it'll exceed 33mm.
Kenda CX Tubeless (not the X-Guard) 32C
After the fact, but we had a small restock.
Since some sold quickly, I got permission from a customer to
mount it briefly on an XR361 rim.
↑Nominally 380g
I couldn't weigh it without a rubber band since
the rubber band weighs less than 1g.
On the XR361 at 2 bar...
Around 33.4mm. That's out of spec.
The Kenda CX Tubeless tends to exceed 33mm when the rim is wider.