Fenix 6 VO2 Max for CYCLING

Just checking, do you have a power meter in your bike and is it paired with your watch?

Hi, no I don’t have a power meter. Do I need one for this?

Yes, cycling VO2max requires a power meter.

Ok thanks, that will explain it! 

I don't think you need power meter for calculating VO2 max. I don't have power meter, but Fenix 6 shows my common and cycling VO2 Max.

I don't even have access to different VO2 max measurements - only one central one within performance stats.

If you just think for 15 seconds, you realize that speed on a bicycle may be quite independent from your effort.

a) you can play with drafting

b) you can play with your posture thus with your body CdA

c) you can play with choosing bicycles with tottally different CdA and Crr

d) on a descent and on an ascent your speed is heavily influenced by the weight of your bike

e) actual wind strength and wind direction vary a lot 

etc etc

If you dont have  a cycling powermeter your Garmin device could use speed together with HR (and maybe HRV) to estimate your VO2max, in the same way as it uses for running. And since speed may be quite independent from your effort, it would be quite stupid to estimate VO2max unless all the factors listed above has been recorded before your ride and/or measured continuously (wind). But even in this case you should avoid drafting and posture changes.

So I am afraid you were totally wrong when you stated one did not need a powermeter for calculating cycling VO2max.

More official sources than me: https://www.garmin.com/en-US/blog/fitness/v02-max-and-aerobic-performance/  

support.garmin.com/.../

Not over 20 km, but more than 20 mins:

Cycling VO2 Max Estimate