Guides · Rim/tyre pairing
Rim inner width: the pairing that decides your aerodynamics
Almost every pressure calculator ignores the rim/tyre pairing. It is precisely the factor that degrades an aero rim's performance the most: when the tyre bulges past the profile, the airflow detaches at that step and a good part of the rim's benefit simply never materialises.
The labelled width is not the real width
Tyres are labelled for a 19 mm internal reference rim (ETRTO). Mounted on a wider rim, the bead spreads and the casing widens: each mm of internal width adds ~0.4 mm of tyre. That measured width — not the label — governs rolling resistance, optimal pressure and frontal area.
| Labelled ↓ · Internal → | 19 mm | 21 mm | 23 mm | 25 mm |
|---|---|---|---|---|
| 25 mm | 25.0 | 25.8 | 26.6 | 27.4 |
| 28 mm | 28.0 | 28.8 | 29.6 | 30.4 |
| 30 mm | 30.0 | 30.8 | 31.6 | 32.4 |
| 32 mm | 32.0 | 32.8 | 33.6 | 34.4 |
Inflated measured width, mm.
The 105% rule
For the rim profile to work, its external width must be at least 105% of the tyre's measured width. Below that, the casing protrudes and creates the step where the flow detaches. You do not choose the external width directly: it derives from the internal one (≈ internal + 6.5 mm on tubeless/hookless rims, whose bead walls are thicker).
| Labelled ↓ · Internal → | 19 mm | 21 mm | 23 mm | 25 mm |
|---|---|---|---|---|
| 25 mm | 1.02 ✗ | 1.07 ✓ | 1.11 ✓ | 1.15 ✓ |
| 28 mm | 0.91 ✗ | 0.95 ✗ | 1.00 ✗ | 1.04 ✗ |
| 30 mm | 0.85 ✗ | 0.89 ✗ | 0.93 ✗ | 0.97 ✗ |
| 32 mm | 0.80 ✗ | 0.84 ✗ | 0.88 ✗ | 0.92 ✗ |
External profile / measured width ratio, tubeless. ✓ = the 105% rule holds. Note how demanding it is against real width: a 28 mm tyre fails on every rim in the table. Some brands compute the rule against the labelled width, which is far more forgiving. Here it applies to the measured width — the one that actually gets mounted.
What a bad pairing costs
A real example from the model: a 60 mm rim with a 28 mm tyre, 75 kg rider at 35 km/h. On a 22 mm internal rim the tyre measures 29.2 mm, the ratio sits at 0.98, and only 78% of the profile's aero benefit materialises. On a 26 mm internal rim the tyre measures 30.8 mm, the ratio rises to 1.06, and the benefit is fully realised. Net difference: 1.5 W in favour of the wide rim — even though its tyre, being wider, costs slightly more frontal area.
That is not small: 1.5 W is more than separates many premium tyres from each other. And the effect is not hypothetical — the calculator's cross-section view shows the exact step where the flow detaches.
ETRTO fitment ranges
| Labelled tyre width | Min internal | Max internal |
|---|---|---|
| 25 mm | 19 mm | 23 mm |
| 28 mm | 19 mm | 25 mm |
| 30 mm | 19 mm | 27 mm |
| 32 mm | 19 mm | 30 mm |
Pairings outside these ranges still compute, but are flagged as out of specification.
Frequently asked questions
Can I fit a 28 mm tyre on a 25 mm internal rim?
Per ETRTO, yes — the range for a 28 mm goes up to exactly 25 mm internal. But check the 105% table: on a 25 mm internal hookless rim (~31.5 mm external), the measured 28 mm (30.4 mm) leaves the ratio at 1.04. Mountable, yes. Aerodynamically clean, no.
Is a wider rim always better?
No. It improves the pairing with wide tyres, but widens the measured tyre, which costs frontal area (~0.0007 m² per mm above 25 mm). What decides the outcome is the ratio with your specific tyre and the pressure you need — which is why the calculator compares complete setups in watts.
I know my rim's external width — can I use it?
Yes: the calculator accepts the measured external width directly. The estimate (internal + 6.5 mm tubeless) is only the starting point when you have not measured it.
Check your pairing
The calculator shows your 105% ratio, the share of the aero benefit you actually realise, and the cost in watts of each combination.
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