Mine is the opposite.

Bike only calculates on trainer because I don’t have a power meter on my bike and I generally only do z1/2 on the trainer so I don’t think it’s entirely accurate.

Differences are expected. Garmin uses HR primary to calculate VO2max besides other metrics like power, pace, time, intervals, etc. and most triathletes are stronger in one of the sports, so the algorithm will produce different numbers on each activity. So, yes you have different VO2max per sport because each sport depends on different muscles and each set of muscles on your body will have different efficiency. Garmin is getting better on the algorithm and it’s a good approximate, but if you want the most accurate number you need to perform a test in a lab where they measure the exact amount of air/oxygen intake during an all-out effort.

It currently shows 54 and 58 in my case. Don't know whether it is meaningful or not.

My numbers are similar to each other (and close to lab testing) but it has my run higher than my bike even though I'm a better biker than runner.

It's likely because I do harder run efforts in training.

I recently started track cycling again and have pushed power and HR very high for me, the gap between the two is closing.

Based on my understanding of how Garmin works :

You have a VO2Max and you have an efficiency. Very simply, your Vo2max * efficiency will give an output, be it power on the bike or pace on the run

The Garmin only sees output so has to make some assumptions on efficiency and work backwards to get VO2max.

The range of efficiency for different cyclists is a much smaller than for running. The range for running efficiency/inefficiency is quite large.

One theory is the Garmin is seeing you are good or average efficiency on the bike and below average on the run. Good news is if this is true, it's "easier" to improve efficiency on the run. "Easy" is a relative term, you just need to run a ton :-)

Allegedly I've got a VO2 max of 63 on the run compared to 51 on the bike, which is absolute garbage.
The Garmin race predictor reckons I could do a 5K in 16:19. My all time best was 19:26 and right now I'm probably in 20:45 shape.
I think the algorithm assumes that as my HR is low for say,4:30/km run pace (because I'm 58 years old), that I could go much faster.

I use Garmin 735 so HR monitor is wrist-based and not the most accurate.

My VO2 on the bike in good shape is over 70. Based on that, I should be running in the 14:XX for 5km, but I'm more of a 17-flat guy (I'd like to run in the 16s, but can't be bothered to put in the 8 week block of Q work to make that a reality). My VO2 on the run charts is more like 60.

A few things: my bike training, when in shape, covers a much larger range of intensities, with a lot of Z4/Z5 and regular TTs etc. I rarely do hard running sets in training (usually just log 45-60mpw with the occasional hill sprints or progression runs), so my Garmin has no idea what my actual fitness is until I race, which has been infrequent the past few years. If I could do 10x3min, 4x4min, 20x1min, 2x20min etc as frequently on the run (without injury) as I did cycling, my VO2 might look more similar, but I still doubt I'd hit my bike numbers. Not built to run.

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This is exactly right. Any VO2max algorithm is going to make implicit assumptions about running economy.

For example, if you run 3:00/km at your VO2max effort and I assume that your running economy is 210ml/kg/km, then I will predict that your VO2max is 70ml/kg/km (210/3). Now, if I'm wrong and you're actually a much more efficient runner of 180ml/kg/km, then your true VO2max could be (180/3) = 60ml/kg/min &, based on pace alone, I would be wrongly predicting your VO2max to be much higher than it is.

As marcag said, this range of possible efficiencies is much narrower on the bike so it can lead to quite a gap, in some people, between bike and run VO2max.

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Agree on the garbage part - it doesn't do multisport athletes very well. Which is odd, since I bought my 945 specifically as a multi-sport watch.

For me, the predictor goes the other way -> its much too low. The highest its ever shown me (peak summer volume) is a 60 VO2 max in running, with a 1:25 half marathon. I've run a 1:21 open HM with the watch when it was showing 1:28 or so. You'd think that it would update when shown a higher value!

All great information - thanks for the replies!

Same for me. 65 bike, 54 run (haven't run much due to a nagging injury).

I find garmin VO2 goes up as HR goes down / flat for the same power.

Wonder if these estimates are accurate... No idea

My running is generally 15 points higher than my cycling. I’m a stronger runner, But not that much stronger. I mainly ride my bike on the trainer and very rarely are the intervals long or hard enough for the number to Update.
On the other hand, I find that the predicted race times are pretty good on the 945, on the 935 they were useless. However, the HM time Seem at bit High, But I rarely run longer than 15-16k for my long runs and usually at a slow pace, so I guess it has no what of knowing How fast I would be. On the other hand, 5k times seem to be within 30 seconds of my actual ability - usually 30seconds faster. That’s Nice for motivation Sometimes on the track, until I give up after 4k gasping for Air

My bike is 72 and run is 58. I guess I have shitty run economy? :)

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What's your CdA?

Wanted to revisit this after I started more structured training on the bike recently (and bought a fan) My Vo2 Max is currently 62 on the bike when back in September it was 55.

I’ve decreased running volume slightly and stopped intervals and it’s down to 58.

I’ve only ever hit 62 on running for a short period when I was in great shape but the way my rides are trending I expect to keep improving on the bike.

Would a nagging knee injury keep me from having my run reach potential high ? I feel great after workouts on the bike and always feel like garbage after a workout on a run.

Garmin's estimates of VO2max are far too variable to be meaningful. Retention of high fitness, especially when doing multiple training modes as in triathlon, is much easier than Garmin's rapid drops in reported VO2max often reflect.

That is, if you cease running but hammer cycling volume and intensity, your VO2max isn't going to go down much at all, even in running, if we were to test it in a lab. Changes in VO2max are not nearly as transient or volatile as Garmin's algorithms seem to think, for most triathletes with more than a couple years of training history.

The Garmin algorithms are too short-term and often ignore years of training history that might influence fitness retention, or rates of fitness reduction, during times of reduced training volume or intensity.

The Garmin algorithms also seem to overlook or underemphasize that the volume or intensity required for fitness maintenance is far lower than the volume and intensity required for fitness improvement. Just because you've dropped to fitness maintenance volume or intensity doesn't mean that all of sudden the algorithm should have your VO2max estimate falling out of the sky. Nonsense.

All that said, I have not yet developed a better system than Garmin so I think it's neat that they've created said algorithms. I just would hesitate to put any weight in those estimates, when it comes to my own training decision-making.

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Well I’m not “trained” compared to most people here.

Last 2 years (45 months) I’ve averaged 6-8 hours a week running. (45-60 miles) a lot of easy miles but I’d go through 2, 3-4 month training blocks a year where I’m building on workouts. Peak winter and summer are mostly easy with 1 tempo a week.

I’ve been biking since July. Was 3-5 hours a week all easy until December now it’s 8-10 a week with SS/Vo2 max reps mixed in.

Now I’m not unfamiliar from training in either discipline as I was competitive in both track and cycling while I was in high school. My 20s was just not spent with much of any aerobic work.

That being said is it feasible my Vo2 max has been this high the whole time, I just am limited in what I can do running do to nagging injuries so it’s “artificially” low on Garmin.

Or is it climbing just because I’ve increased volume in terms of hours a week I’m working out? Previously 8-9 now 14-15.

Agreed, a lot of variability. It does not appear that Garmin factors in changes in ambient temperature in its calcs.

A heat wave basically guarantees that Garmin will drop my VO2max, since my HR will be higher at the same power output. A cooler week and it pops right back. Ditto for a lot of Zwifting; it gets me in great shape, but the VO2max doesn't move until I use the fitness to lay down some outdoor workouts.

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As far as I know the newer Garmin devices with heat and altitude acclimation features do take into account these variables when displaying VO2max.

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Strava

You have plenty extensive enough of a training history not to have a 7-8% drop in your real VO2max as tested via running. Garmin is just wrong, and I wouldn't attempt to make meaning out of it, in your case, or in anyone's case, to be honest.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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I'm running a Fenix5 (baro altitude and temp sensors, latest firmware) and still seem to have the temp-based VO2 variability issues on the watch and Connect.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I also don't rely too much on the Garmin nr's.
That being said; what I did notice is that after I put in the exactly observed HR-limits as found in my Labtest (with oxygin mask) that now my Garmin (945) is very close to these lab-nr's...
bike : 66.5 (lab) vs 67 (garmin)
run: 64.2 (lab) vs 65 (garmin)
* these were not the true VO2max numbers as observed during the tests but the highest average VO2 numbers (I think max 30sec avg)

Furthermore I also notice that during summer the numbers on the Garmin seem to be way off indeed (esp. run drops rapidly with temp.)
finally the race-predictor time ... that one's indeed fishy, for some reason it can just predict a slower time than I just did in a fast run (esp noticed for HM; but never truly did any run-race (yet...))

Resurfacing this thread as I often wondered how accurate the Garmin VO2 Max numbers were.

I recently did bike and run tests with the blood lactate measurement and facemask. My Garmin says I have a cycling VO2 max of 57 but the test came back as 50.5. My running VO2 max is 52 on the watch but is actually 57 according to the test. So, basically, the watch had the two numbers pretty much opposite.

Just thought i'd share that in case anybody else was wondering about the numbers on their watch.

Interesting. My VO2Max numbers (from Garmin) are similar to yours. I think that the value of the Garmin estimates are the relative numbers as you move through time. For that, and for me, the correlation seems quite good. I've had a few injuries over the last 18 months or so and the VO2Max numbers from Garmin seem to track how I feel as the training stress ebbs with injury and returns with recovery. I've been focused on cycling for the last couple of months and my Garmin VO2Max there is good, but my running training has suffered (broken leg and all that), so my running VO2Max from Garmin is somewhat below the cycling VO2Max estimate. To me, it mostly seems sensible, but what my experience lacks is the absolute values generated in the lab for comparison (that you have). I'd like to get a lab based VO2Max done at some point, but I'm actually surprised at how well the Garmin VO2Max predictions seem to track for me as 'relative' measures of fitness.

Have you done maximal efforts while using your watch?

That may be the difference.