Alto del Arenal

Overview

Alto del Arenal is an outlier even among hors catégorie climbs. At 55.4 km long it is not a climb you attack — it is a climb you survive. The road lifts from a start elevation of 625 m to a summit of 3788 m, a net elevation gain of 3163 m over a single continuous effort. Because the profile pitches and eases rather than rising in a clean line, the total ascent reaches 3363 m, meaning there is roughly 200 m of extra vertical hidden inside the dips and false flats.

The average gradient of 5.71% sounds forgiving on paper, but averages lie on a climb this long. Stretched across 55.4 km, that number blends brutal ramps with recovery zones, and the max gradient of 20.87% tells you the walls are real. This is a full HC ascent — the hardest category on the road — earned by both its length and its raw vertical.

How the Gradient Unfolds

Think of Alto del Arenal in thirds. The lower slopes trend near or below the 5.71% average, letting you settle into rhythm from 625 m without immediately drowning. As you climb, the gradient becomes less patient: the middle section is where the pitches toward that 20.87% maximum bite hardest, often stacked back-to-back with only short reprieves.

The upper third is defined less by gradient and more by altitude. Approaching 3788 m, thin air flattens your ceiling regardless of how steep the tarmac is. A 6% ramp near the summit costs far more than the same ramp near the bottom. The 200 m gap between net gain (3163 m) and total ascent (3363 m) confirms the road is not monotonic — expect brief descents and flats that break your cadence right when you have found it.

Pacing Notes

On a climb of 55.4 km, discipline beats ambition every time. Anchor your effort to your sustainable threshold. Riding just below your FTP on the lower and middle slopes preserves the reserve you will need as altitude compounds the effort near 3788 m. If you do not know your threshold, calibrate first with the FTP estimator so your target watts are honest rather than optimistic.

Treat the steepest ramps toward 20.87% as brief overshoots, not new baselines — surge, crest, and settle back down. Never chase the average of 5.71% as a fixed power target; let gradient dictate cadence and keep your effort even.

Gearing

Gear for the walls and for the altitude, not for the average. The 20.87% maximum gradient demands a compact setup with a wide-range cassette. A 34×32 is the sensible floor for most riders, but given the sheer 55.4 km distance and the fatigue of climbing to 3788 m, a 34×34 is the smarter choice — the extra sprocket buys you a lower cadence option precisely when your legs are emptiest on the upper third. Err toward the easier gear; you will never regret spinning up a 20.87% ramp instead of grinding it.

Summary

Alto del Arenal is a rare monster: length, verticality, and altitude fused into one HC effort. Pace with restraint, gear low, and respect the 3163 m of net climbing between you and the summit.

Length: 55.4 km
Average gradient: 5.71%
Max gradient: 20.87%
Start: 625 m
Summit: 3788 m
Net gain: 3163 m
Total ascent: 3363 m
Category: HC