Col de la Boîte aux Lettres

Overview

The Col de la Boîte aux Lettres is short on paper and merciless in reality. Over just 4.8 km it drags you from a start elevation of 1323 m to a summit of 2165 m, a net gain of 842 m packed into a distance most riders would consider a warm-up. That works out to an average gradient of 17.46%, which places it firmly in the hors catégorie (HC) bracket — the hardest classification, above categories 1, 2, 3 and 4. This is not a climb you settle into; it is one you survive.

How the Gradient Unfolds

There is nowhere to hide on a climb that averages 17.46%. With 842 m of net elevation gain across 4.8 km and a total ascent of 825 m, the road offers almost no relief — the small gap between net gain and total ascent tells you there are barely any downhill or flat metres to recover on. Expect the gradient to sit relentlessly in the high teens for long stretches, the kind of slope where your front wheel feels like it wants to lift and your cadence collapses regardless of effort. From the 1323 m base to the 2165 m top, every one of those 842 vertical metres is earned the hard way, with the thinning air near the summit adding a final insult to already screaming legs.

Pacing Notes

On a wall this steep, pacing is less about tactics and more about not blowing up in the first kilometre. Because the average never dips, your power will spike well above your sustainable ceiling the moment you stop concentrating. Ride to your FTP rather than to feel — the gradient will happily lure you into surging, and surging on a 17.46% slope is how a 4.8 km climb ends your day. If you do not know your threshold, run the numbers through the FTP estimator before you go anywhere near this col, then aim to hold a steady, repeatable effort from the 1323 m start. Micro-recoveries are scarce given how little the total ascent of 825 m differs from the 842 m net gain, so treat the whole 4.8 km as one continuous effort.

Gearing

Gear for survival, not for style. On a climb averaging 17.46% with 842 m to gain, most riders will want the easiest setup they can bolt on. A 34×32 is the bare minimum here and will have you grinding at low cadence for much of the 4.8 km; a 34×34 gives you a little more room to keep the pedals turning and your knees intact. If you can fit anything easier than a 34×34, do it — there is no shame in spinning up an HC wall, and preserving cadence to the 2165 m summit will save you far more than a heroic gear ever could.

Summary

The Col de la Boîte aux Lettres is a pure test of steepness: short, unforgiving and unambiguously HC. Bring your lowest gears, your patience, and a plan to hold threshold from bottom to top.

Length: 4.8 km
Average gradient: 17.46%
Max gradient: n/a
Start: 1323 m
Summit: 2165 m
Net gain: 842 m
Total ascent: 825 m
Category: HC