Pace, speed and GAP: how to compare training sessions on hills
Pace (min/km) is the most used metric in running — and the most misunderstood. It is valid as long as the terrain is comparable, and it stops being valid as soon as the route has elevation gain.
Pace and speed are not the same scale
Pace is time per kilometre; speed is distance per hour. The difference matters because the relationship is not linear: going from 6:00 to 5:00 min/km means gaining 2 km/h, while going from 4:00 to 3:00 min/km means gaining 5 km/h. When you compare the "average pace" of two sessions you are really comparing its inverse, which exaggerates differences at faster paces.
Why elevation changes everything
Uphill, the energy cost of each kilometre rises sharply: for every 1 % of positive gradient, metabolic cost increases by roughly 3–4 %. Downhill, the saving is not symmetrical: even with the same gradient, you do not get back everything you lost on the way up.
Practical consequence: comparing your average pace on a flat long run with a mountain route will normally show a 30 to 60 second per kilometre gap that does not mean you are less fit. You simply did more work.
What GAP (grade adjusted pace) is
GAP is your pace adjusted for gradient: it estimates the flat-ground equivalent pace for each section of the route. It is the same calculation behind grade adjusted distance (NGP or "equivalent distance") and it lets you answer the right question: was this session harder than last week's, or just hillier?
| Section | Actual pace | Elevation | Estimated GAP |
|---|---|---|---|
| Km 1 | 5:10 /km | 0 m | 5:09 /km |
| Km 2 | 6:05 /km | +80 m | 4:48 /km |
| Km 3 | 4:35 /km | −75 m | 4:55 /km |
Look at Km 2: although the raw pace looks slow, adjusted for gradient it was the fastest kilometre of the three. Without GAP that effort stays invisible.
How to use it without overthinking
- To compare sessions: use GAP when elevation gain exceeds 10 m/km. Below that, GPS noise outweighs the gradient.
- To plan hill intervals: forget target pace and work by effort, heart rate or power. GAP is for reviewing afterwards, not for guiding the session.
- To calibrate your pace zones: if your "easy runs" are hilly, GAP will tell you whether they were genuinely easy or a tempo in disguise.
- About GPS: barometric altitude is more stable than GPS altitude. If your watch measures it, enable altitude correction.
See it in your own activities
In Afterpacer every activity shows the elevation profile alongside splits and heart rate, so you can cross-check all three: if an uphill kilometre has a higher heart rate and a faster GAP, you know exactly what happened.
Open the sample activity without signing up, or upload your own GPX file to compare with your real data.