Here some research based on my experiece in this rather complicated matter:

I found that for me the VO2max value on my FR 935 corresponds very well with my VO2max lab result. However, Garmin’s Race Time Prediction shows values I’m unable to reach.

Digging a little deeper, I found the following whitepaper by Firstbeat:

https://assets.firstbeat.com/firstbeat/ ... 6.2017.pdf
For the Race Time Prediction, the Firstbeat algorithm in Garmin's watches just uses Jack Daniels' Race Time Prediction Table (Table 4 on page 7), which Firstbeat calls "Race time prediction based on VO2max".

There lies the misunderstanding: Jack Daniels does not use VO2max. He uses his own concept called VDOT, which is not the same! The scales are different. *)

Runalyze correctly labels their own concept “effective VO2max”, because it is "reduced" by some factors regarding your running efficiency.

Therefore, Runalyzes “effective VO2max” is much more similar to VDOT than to VO2max. I found that their prognosis also matches my race results very well.

Garmin determins VO2max more or less successfully, but this value alone is useless and inserted into unsuitable (VDOT-)tables produces wrong results.

*) Comparison between VO2max and VDOT:

Take the Cooper test:

- If you run 3 km in 12 minutes, you will get a VO2max of 55.8.

- If you run 3.219 km (2 miles), you will get a VO2max of 60.7.

*(Formula: (Distance covered in meters - 504.9) ÷ 44.73)*
Using Jack Daniels' tables (

https://www.coacheseducation.com/endur/ ... nov-00.php ) for the examples above:

- For 3 km in 12 minutes, you'll get a VDOT of 47.9.

- For 2 miles in 12 minutes, you'll get a VDOT of 52.1.

This shows that VO2max and VDOT values are not interchangeable! For both distances, VDOT values are only about 85 % of the VO2max values. So if you have a VO2max of 56 and a VDOT of 48, the Garmin/Firstbeat algorithm will look into the VDOT table with 56 instead of 48 and therefore get a much too optimistic race prediciton.