Which Race Predictor Is Right: Strava or Your Watch?
Garmin, COROS and Strava predict your race times in completely different ways. A 1977 formula helps you decide which to trust.
I was playing around with my data this weekend, and a question popped up: why are these predictions so different, and which one is right?
They almost never agree, because they don't work the same way at all.
Three ways to predict a race
Garmin and COROS predict off your estimated VO2 max.
Strava uses machine learning on your all-time history and compares you to athletes like you. It explicitly does not use VO2 max.
The traditional method uses a simple formula developed in 1977 by Pete Riegel.
The Riegel formula
Time₂ = Time₁ × (Distance₂ / Distance₁) ^ fatigue factor
The fatigue factor is traditionally 1.06, but some athletes are better adapted to certain distances, so it varies from person to person.
Find your personal fatigue factor
I built a Claude prompt that compares old-school and new-school predictions. It takes your actual race results, solves for your personal fatigue exponent (how much you fade as distance goes up), puts that next to what your watch says, and tells you which one to believe.
A fun weekend project for anyone who likes the math.
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