Gravel Tyre Width Guide

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Start with the finding, because it’s the reason this page exists. We took the six gravel tyres in our tyre roundup that Bicycle Rolling Resistance has measured, put the labelled width next to the measured width, and got this:

Six labels. Six measurements. All six came in narrower. The Panaracer labelled 40 measured 37. The Schwalbe labelled 40 measured 36. The Teravail labelled 42 measured 38. The WTB labelled 37 measured 35. Not one tyre in the group made its number.

That is not six manufacturers cheating, and the explanation is more useful than an accusation would be.

The number on your tyre is a nominal width, not a measurement

Under ISO 5775 — the international standard that governs bicycle tyre designation — a tyre marked 40-622 carries a nominal section widthof 40mm on a 622mm bead seat diameter. “700x40” is the same tyre in the older French notation. The key word is nominal. It is a designation, not a promise about the object in the box.

The standard is explicit that realised width depends on the rim: it specifies that rim inner width should be roughly 55% of nominal section width for tyres over 30mm (and 65% for tyres under 30mm). Mount a tyre on a rim narrower than its design rim and the casing pinches in and measures narrow. Mount it on a wider rim and it spreads and measures wide. Same tyre, different number.

Which means the honest caveat on our own table: BRR measures on one rim.Its published GP5000 measurement notes an inner rim width of 17.8mm, and plenty of modern gravel rims are 21-25mm internal. On a wider rim, those same tyres would measure closer to — or past — their labels. So the deltas below are BRR’s deltas on BRR’s bench, not a law of nature. The directionwas consistent across all six, and that’s what you should plan around.

How wide should you actually go?

The instinct is that narrow is fast, so go as narrow as you can bear. The published evidence says that instinct is mostly wrong.

Rene Herse’s roll-down testing compared 28, 32, 35 and 44mm tyres in the same casing on a 132m hill and found the differences were not statistically significant — 44mm tyres were not slower than 28s, even at 29 km/h where aerodynamics start to bite. The mechanism they give is that a narrow tyre at high pressure has low hysteresis loss but high suspension loss (energy lost shaking you and the bike), while a wide tyre at low pressure reverses both, and the two roughly cancel.

Rene Herse manufactures and sells the tyres in that test, and won its own test. We say that every time we cite them, because you deserve to weigh it. What makes the result credible anyway is that the mechanism is coherent and the method is published — go read it and disagree with them if you like.

So if width is roughly free in watts, the answer becomes: go as wide as your frame clears, minus margin.The costs of width are real but they aren’t speed — they’re weight, clearance and mud.

Casing matters more than width, and nobody publishes it

Here’s the uncomfortable part. In BRR’s figures the spread between the fastest and slowest tyre in our roundup is 20.9 watts to 26.0 watts — about a quarter — and those tyres are all 37-40mm. Width didn’t cause that spread. Casing and compound did.

The same point shows up in the puncture scores, which range from 24 points to 35 across tyres that look identical in a product photo. The WTB Riddler took the lowest tread andsidewall puncture scores BRR had recorded at that point. The Teravail Cannonball is sold in multiple casings — BRR tested the Light & Supple and flagged sidewall punctures as a potential deal-breaker, while the listing we link is the Durable. Same tread, same brand, different tyre.

And yet: not one of the seven listings in our roundup publishes a TPI count, and not one publishes a weight.The spec that decides most of the tyre’s behaviour is the spec no seller prints. That is worth being angry about, and it’s why we put a “Not published” column in the roundup rather than filling it with guesses.

Tubeless is what makes width worth having

Wide tyres pay off at low pressure — that’s the whole mechanism above. Low pressure with an inner tube gets you pinch flats. So the width conversation and the tubeless conversation are the same conversation, and six of the seven tyres in our roundup state tubeless on the listing because the category has already decided.

Rene Herse publishes one genuinely useful rule of thumb for pressure: every millimetre of rim inner width over 19mm reduces recommended pressure by about 0.2%. Small, but it points the right way — wider rim, wider tyre, less pressure. Beyond that they decline to publish a formula and refer you to their calculator, which is honest of them and mildly annoying if you wanted the maths. We’ve set out how to find your own number in the tyre pressure guide, and the honest costs of the tubeless system in tubeless vs tubes.

Clearance: buy margin, not the maximum

Trek publishes 50mm clearance on the Checkpoint ALR 5 Gen 3 — and 42mm with fenders fitted. That eight-millimetre fender tax is the kind of thing that turns a confident purchase into a rubbing tyre in November.

Stack the two effects and you get the practical rule. Your tyre may arrive up to 4mm off its label in either direction depending on your rim. Mud fills whatever gap is left. Fenders eat 8mm. So buy to your published clearance minus about 4mm, and if you run fenders, work from the fendered number. A 45mm tyre in a frame published at 45mm is not a 45mm tyre in a 45mm frame — it’s a coin flip.