Are the race predictions reliable?

Its a software estimating from your HR and rest and other metrics. It doesn’t know anything about your real body so I would take that with lots of doubt.

On my Fenix 5X it’s said a full marathon in under 3 hours... if I were to try that my body would end up at the hospital!

Now with the Tactix Delta (Fenix 6) it’s says 4 hours 26 minutes and that’s more realistic, so at least the metrics are night and day from the Fenix 5 and 6 (software? Hardware?)

So maybe an answer to your question? More reliable now than ever, but still I doesn’t know anything about your mind and body/skeleton/ligaments/muscles/genes etc.

Not really. 

Miky said:

Has anyone already checked whether these predictions are reliable?

It's well known that the race predictions are wrong.

https://hetgeheimvanhardlopen.nl/en/what-is-wrong-with-the-garmin-race-predictor/

Please have a look at Table 4

https://assets.firstbeat.com/firstbeat/uploads/2017/06/white_paper_VO2max_30.6.2017.pdf

Firstbeat confused VO2max and Jack Daniels VDOT.

Here's what my race predictor currently says (I'm 45, 65 kg, VO2max: 55):

5k: 19:12

10k: 40:31

Half: 1:30:29

Marathon: 3:16:29

And here are my PBs:

5k: 20:14

10k: 40:42

Half: 1:28:06

Marathon: 3:20:30

The 5k PB was achieved as part of a much longer race, so in reality if I raced a 5k distance, I'm convinced I could get significantly faster than that. The half-marathon PB is very recent, so it is the most accurate relfection of my current fitness. The marathon PB is 4 years old, and I'm quite sure that today I could run it slightly faster.

So in my case, I'd say the race predictor is nigh perfect. Your mileage my vary, though.

Great news!

A significant factor in VO2Max (and therefore race predictor accuracy) is how accurate your estimate of HRMax is. If your HRMax is overestimated, the algorithm will think you are cruising and capable of going a lot faster, whereas in fact you may be close to your limit already.

Definitely worth considering how confident you are that the HRMax figure in Garmin Connect is close to your true personal maximum.

Good advice. In a run I've recorded 190bpm and so that's my input for HRMax (It's easy for me go over 180bpm in a run at 5:20/km, 190bpm were recorded during a training with multiple 800m at 4:40/km)

RunningM said:

On my Fenix 5X it’s said a full marathon in under 3 hours... if I were to try that my body would end up at the hospital!

Now with the Tactix Delta (Fenix 6) it’s says 4 hours 26 minutes and that’s more realistic, so at least the metrics are night and day from the Fenix 5 and 6 (software? Hardware?)

So maybe an answer to your question? More reliable now than ever, but still I doesn’t know anything about your mind and body/skeleton/ligaments/muscles/genes etc.

In the F5 generation, the race predictor only took into account VO2Max. The F6 generation takes into account recent mileage as well as VO2Max.

For example, assume you have been training hard for a 5K, and producing a good VO2Max figure. The F5 algorithm would take this and estimate a fast marathon. The F6 would see that you have only be doing 5Kish distance and produce a fast time for that distance, but a much slower estimate for the marathon, as without the long distance mileage, it will assume you are going to run/walking the marathon. 

Mirko, if you are confident of your HRMax estimate, it may be worth trying a few intervals at that faster race pace, and seeing if that pace feels sustainable. If they do great, worst case is they don't and you stick to your 50 minute plan.