Alto del Mirlo

Overview

Alto del Mirlo is a long-haul category 1 climb that rewards patience over punch. It runs for 31.3 km from a valley floor at 434 m up to a summit of 1755 m, banking 1321 m of net elevation gain along the way. The total ascent of 1318 m confirms there is almost no let-up: this is very nearly all up, with only a handful of metres given back to descents. The headline average of 4.22% sounds gentle, and for long stretches it is — but a maximum pitch of 14.4% means the road has real teeth hiding inside that mild number. At 31.3 km, this is a climb measured in effort budgeted, not attacked.

How the Gradient Unfolds

Do not be fooled by the 4.22% average. On a climb this long, the mean is a blend of shallow drags and sharp ramps, and the 14.4% maximum tells you the two extremes are far apart. Expect the gradient to breathe: shallower valley sections where you can hold rhythm and momentum, then steeper walls where the road rears up toward that 14.4% pitch and forces a gear change. With 1321 m to gain across 31.3 km, the sustained sections do the bulk of the work, while the steep ramps are where time and matches are lost. The near-parity between the 1321 m net gain and 1318 m total ascent means you climb almost continuously — there is little recovery coasting to bail you out between the hard bits.

Pacing Notes

The single biggest mistake on a 31.3 km climb is starting too hard. Set your effort against your FTP and hold it — the length means small overefforts early compound badly by the summit. Aim to ride the shallow sections just under threshold, then accept you will spike above it briefly on the 14.4% ramps rather than trying to hold a fixed pace through them. If you do not know your threshold, run the numbers through the FTP estimator before you go. On a climb gaining 1321 m, a disciplined, even effort will beat a heroic start every single time.

Gearing

With a 4.22% average stretched over 31.3 km and pitches to 14.4%, gearing is about protecting your legs for the long haul, not just surviving one wall. A compact 34-tooth inner paired with a 34×32 cassette gives most riders enough range to spin the shallow drags without grinding. But because the steep ramps hit late — after you have already climbed much of the 1321 m — the extra safety of a 34×34 is well worth it. If your knees or your endurance are in any doubt on a climb this long, fit the 34×34 and enjoy having a bail-out gear in reserve.

Summary

Alto del Mirlo is a test of pacing and gearing over raw power: long, relentless, and steep exactly where it hurts most. Budget your effort, keep a low gear spare, and let the 31.3 km come to you.

Length: 31.3 km
Average gradient: 4.22%
Max gradient: 14.4%
Start: 434 m
Summit: 1755 m
Net gain: 1321 m
Total ascent: 1318 m
Category: 1