How Running Economy is estimated. Garmin's own figure needs an HRM 600 strap (it measures step speed loss), so this is an approximation on the same scale — ml O₂/kg/km, lower is better. Oxygen cost is estimated from %heart-rate reserve (the standard %HRR≈%VO2R relationship) and divided by speed.

Every run is measured over a fixed km 2–4 window, not the whole run. Measured end-to-end, the number tracks how far you ran rather than how efficiently: longer runs accumulate cardiac drift, and on your history that produced a false decline exactly while your VO₂max rose. Treadmill runs are excluded (their pace is accelerometer-derived), as are intervals, hilly runs, and anything outside an easy/steady effort — the same conditions Garmin requires.

The score is relative to your own baseline (50 = your median), because the absolute value carries an unknown offset. Trust the rolling median line over any single run.

Every card has a “How it’s computed” toggle with its definition and formula. Full reference, including all filters and caveats: docs/performance-metrics.md.