How PIRX Differs From Garmin
Why your PIRX Projected Time and your watch disagree
If your Garmin (or Coros, Polar, or Apple) race predictor shows a different time than your PIRX Projected Time, that is expected. The two numbers come from different methods and answer different questions.
What your watch predicts
Most wearable race predictors are VO2max-derived. Your watch estimates your VO2max from heart rate and pace during runs, then maps that estimate onto a population race-time table. Because VO2max models describe a physiological ceiling -- the best you could theoretically run on a perfect day -- they often run optimistic, especially for longer events and for runners who have not raced recently.
What PIRX projects
PIRX is race-anchored. Your Projected Time is built from:
- Your actual race results and hard training efforts, scaled across distances with an individual power-law fit (Riegel / Local Matrix Completion).
- Your recent training -- volume, threshold density, speed exposure, and load consistency over the last several weeks.
PIRX projects what your current paces and training support right now, not a physiological ceiling. When your training dips (for example, after an injury), PIRX reflects it; a VO2max estimate often will not, because VO2max changes slowly.
So which one is right?
They are answering different questions:
| Wearable predictor | PIRX Projected Time | |
|---|---|---|
| Based on | Estimated VO2max | Real races + recent training paces |
| Describes | Physiological ceiling | What your training currently supports |
| Tends to | Run optimistic | Track your real, recent form |
Use your wearable number as a rough ceiling, and use PIRX to understand what you can run today and how your training is moving you toward your goal. If you want PIRX to recognize a faster capability, log a race or a hard, sustained effort -- that gives the projection a new anchor.