Forerunner 255 VO2 max? what's wrong?

That is correct and the things you mention should probably affect V02 max. But I have not changed any of the patterns for my training and heart rate with the new watch seems to be measured even a bit lower, which makes the massive downturn even more strange. Of course there could be physilogical explanations, but I doubt it, since the steep downturn started exactly when I started to use the new watch.

Have you checked that all your stats (Max HR, age, weight, etc.) are correctly set in the new watch? They probably should copy over from the old one, but it's worth a look because that could explain the drop in VO2max.

Just my 2 cents, but I think max HR is the most important (and maybe only) physiological parameter that affects VO2 Max calculations for Garmin devices.

People always say that other stats such as age and weight need to be correct, but I've never bought that. Obviously age and weight affect your VO2 Max, but that doesn't mean they are used in the calculation for VO2 Max. My understanding of Garmin's VO2 Max calculation was always that it looks at your speed and HR data for a sub-maximal effort (throwing out data that isn't applicable, such as being stopped at red lights), and extrapolates to a maximal effort using your max HR. Then it uses a lookup table to determine your VO2 Max (maybe some variant of the Jack Daniels VDOT lookup table). This is based on the FirstBeat whitepaper for VO2 Max, which also says that other physiological parameters are *logged* (but says nothing about actually using them in the calculations).

(This isn't too dissimilar to those online VO2 Max / VDOT calculators which ask for a race result - time and distance - and spit out a VO2 Max based on that, except for the parts about throwing out inapplicable data and extrapolating from a submaximal effort to a maximal effort. Note that such "race result to VO2 Max / VDOT" calculators ask for no other information than race distance and time.)

In other words, if 2 people of different ages and weights have the exact same max HR, with the exact same pace and HR for the exact same distance, I would expect their calculated VO2 Max to be the same. I wouldn't expect age or weight to play any role in the calculation. Same goes for gender (another parameter that may *affect* VO2 Max in terms of population averages, but should seemingly play no role in Garmin's VO2 Max calculation).

As a concrete example, if 2 people of differing age, weight, height and gender happen to run a 10k race at max effort in the exact same time (e.g. 42:00), then I would say they have the same VO2 Max (or rather, the same VDOT / effective VO2 Max [*]). Similarly, if 2 people happen to have the same max HR and run 10k with the exact same HR for the same time, then I would expect Garmin to estimate the same max HR.

I could be wrong though. A FirstBeat employee (HermoT) once commented in these forums that physiological parameters such as weight are important to the algorithm, but I never really understood how.

I guess I could always tweak my weight significantly in Connect and see if my VO2 Max changes significantly (all other things being equal), but I've always been reluctant to do that test. Personally I don't think it should (if I actually lost a ton of weight, I should be able to run faster, all other things being equal, and therefore have a higher VO2 Max, but if I just *say* I lost a ton of weight without running any faster, why should my VO2 Max increase?)

I have gained about 5 lbs suddenly and recently (and Connect knows about it), but my VO2 Max has continued to increase steadily (as it has been doing for about 3 months). I realize that proves nothing ofc. However, the increase itself makes sense, as over that time, I've been running at same pace or faster but my average heart rate has been decreasing.

[*] VDOT is Jack Daniel's term for "VO2 Max" as determined/estimated by a race result. He uses this special terminology because it's not really your literal VO2 Max (which is a measurement of how much oxygen your body can use). In reality, 2 people who run a 10k race at the same speed can have differing VO2 Max values, with differing running economy closing the gap (for example). But with the Daniel's method of estimating VDOT from race results, 2 people who have the same race result will always have the same VDOT.

Similarly, "effective VO2 Max" is runalyze's term for the "VO2 Max" value that's estimated from speed and HR data (supposedly in much the same way that Garmin does it). 

As far as I understand, VDOT is a basic indicator of run result as only consider distance and pace and you can get increased values only if you run faster, without attention to your heart rate. What worth this to? 2 people covering the same distance with same time but different HR, have same VDOT but different VO2max.

I am checking and record my VO2max after every runs, and I am stable at 46 while some weeks ago I was 44. The increase from 44 to 46 due to my HR max decreased, and my HR is sensibly decreased of ca. 7-8 units running at 5:40, imho.

First of all, that was more than 2 cents! :-)

But I think you are missing out on one important thing regarding weight. The unit for VO2Max is mL/kg/min. That means your VO2Max is inversely proportional to your body weight.