What is wrong with the Garmin Race Predictor?

What is wrong with the Garmin Race Predictor?

Many times runners and coaches have asked us how it is possible that the Garmin Race Predictor   predicts way too fast times, which are absolutely impossible for many runners. There is also a lot of criticism on the internet from disappointed runners, who are demotivated by the predictions of the Race Predictor. That is all the more unfortunate as most runners hope and expect that their watch can make a good prediction of race times based on the training data. That is an important reason to buy such a watch!

We took a good look at this. Your Garmin first calculates your oxygen uptake VO2 max based on your training data, especially your speed and heart rate data. The Race Predictor then predicts your race times based on this VO2 max. How does this work and what are the results in practice? And what is the reason it that the predicted times are too fast? As always, we studied the theory and also tested our own results in practice with Garmin watches (Hans uses the Forerunner 935 and Ron the Fenix ​​6X).

How does it work? VO2 max

 One of the nice aspects of a running watch is that you get an update of your fitness every day in the form of your VO2 max. The watch calculates this from your training data and in particular the data of your speed and your heart rate. As we previously showed in our articles, the VO2 max is a very useful measure of fitness: the higher your VO2 max, the better your fitness and the faster you can run.

The table below shows the standard classification of VO2 max values ​​of men and women as a function of their fitness (which is also used by Garmin):

How fast can you run with your VO2 max?

In principle, your running speed is directly proportional to your VO2 max. In practice, the weather conditions and the course and distance will of course also have an impact. All these factors are covered in our books. You can also calculate them yourself with our calculators. The race time at various classic distances is shown in the table below (which applies to ideal conditions).

How does it work? Garmin’s Race Predictor

Garmin’s Race Predictor predicts race times at 5K, 10K, half marathon and full marathon based on your VO2 max. How this works exactly is a Garmin trade secret. In principle. Garmin takes into account the personal information (especially age and gender), the training history and training structure and the data of the training (especially heart rate and speed, divided into different segments).

On the internet we found a useful and interesting overview of the relationship between Garmin’s VO2 max and the predicted times of the Race Predictor.

The first 2 columns of the table below are taken from this overview and show the relationship between the Garmin VO2 max and the predicted 5K time. Next we took that predicted 5K time as a starting point and calculated with our universal running model (as explained in The Secret of Running) which “real” VO2 max corresponds to that time. The result is in column 3 and appears to be no less than 3-5 units higher than the VO2 max reported by Garmin. The difference between the “real” VO2 max and Garmin’s is in the 4th column.

How it is this possible? An analysis.

The results of the table correspond to the experiences of many runners and coaches and can also be seen in numerous posts on the internet. Usually it is reported that you only get realistic times if you subtract 3-4 units from the Garmin VO2 max. Various running friends have reported to us that they also get realistic times if they take the Garmin VO2 max and use our calculator (and so not the Race Predictor). Coach Sander Schreurs even sent us an anonymized Excel with the VO2 max and Race Predictor values ​​of his athletes running with a Garmin.

We can illustrate this with the results of author Hans (marked in yellow in the table). Hans has his VO2 max regularly determined at the physiological lab of SMA Midden Nederland and therefore knows that his VO2 max is in the order of 60 ml/kg/min (depending on the shape of the day of course). His Garmin Forerunner 935 watch neatly shows the same value (depending on the shape of the day it varies slightly between 59 and 61). Hans runs times of around 18:30 at the 5K, which corresponds well with the result of our calculator with a VO2 max of 60. However, the Garmin Race Predictor predicts an unthinkable time of 17:01! Hans would need to subtract 4 units from his Garmin VO2 max of 60 to get a realistic time of 18:30.

As an aside, we remind you that Hans is currently injured and has therefore started cycling. Remarkably, since he started cycling six months ago, his Garmin watch has steadily increased his cycling VO2 max to a value of no less than 65! This is strange, because Hans has been running for 40 years and has only been cycling for six months, so why would his cycling VO2 max be so much higher? But even stranger is the fact that the Garmin has simultaneously increased its running VO2 max to 65! And the Garmin Race Predictor now predicts a 5K time of 15:53! That is completely surreal, Hans ran this time 35 years ago when he was around 30. For the M65, this would mean a new world record!.

All this has made Hans curious whether his cycling VO2 max is really so high or whether Garmin’s cycling algorithm is incorrect. He therefore plans to do a bicycle test at the SMA to determine what his cycling VO2 max really is. So to be continued.

How is this possible? A possible explanation.

Without information on Garmin’s exact algorithm, it is of course very difficult to explain what is wrong with Garmin’s Race Predictor. On the internet, however, we found the original patent of FirstBeat Technologies (Garmin’s partner).. It states that the oxygen consumption during running is calculated with the formula: VO2 max = 3.5*v, where v is the speed in km/h.

With our universal running model, however, we have deduced that the correct formula for oxygen consumption (without air resistance) is: VO2 max = 3.77*v. This corresponds to a difference of over 7%. Consequently, at the same value of the VO2 max, Garmin calculates a considerably higher speed v and thus a faster time. In addition, the air resistance, which is greater for faster runners, must also be taken into account. We have not been able to find whether and how Garmin calculates this.


Garmin’s Race Predictor systematically predicts way too fast times.

The explanation for this is probably an error in the algorithm that calculates the speed/time. We believe the VO2 max of Garmin is realistic.

There are 2 solutions to this problem:

  1. Use the VO2 max of Garmin (or that of a test at a physiological lab) in combination with our calculator (this gives the correct times)
  2. Decrease Garmin’s VO2 max by 3-5 units in combination with the Race Predictor (the order of magnitude will be correct depending on where you are in the table).

An example of too fast race times by the Garmin Race Predictor

Our book ‘The Secret of Running’ is for sale in our webshop. Also available in German as ‘Das Geheimnis des Laufens’, and in Italian as ‘Manuale completo della corsa’.