My Garmin once told me my VO2 max was 61, high enough to put me in the purple, the top tier on the watch.
Then I checked the race predictor on the same watch, and it said my best marathon would be 2:40:55.
My actual marathon best is 2:16:59, and I haven’t lost that much fitness.
So, if your watch says your VO2 max is dropping, or that you’re slower than your race results, how much should you trust it?
To answer that, here’s the research-backed practical advice on:
- How close Garmin’s VO2 max estimate gets to a lab test
- Why your number drops after hot runs, hills and trail runs
- What predicts your race times better than VO2 max
- And how to set your training paces without relying on the watch
How Accurate Is the VO2 Max on Your Garmin?
Garmin’s VO2 max estimate is close to a lab test for a group of runners on average, but it can be far off for one runner.
A 2022 analysis pooled 14 studies that compared VO2 max estimates from consumer wearables with lab tests.
Watches that worked out VO2 max from heart rate and other data recorded during exercise were right on average, but for any one person the estimate could land about 10 ml/kg/min above or below the lab result.
That means a watch reading of 50 ml/kg/min could belong to a runner who tests at 40 ml/kg/min in a lab, or one who tests at 60 ml/kg/min.
Newer watches still miss on race times.
A 2026 study of the Garmin Forerunner 265 found its half marathon predictions were about 7.5 minutes faster than the times runners ran after 12 weeks of training.
The same watch put runners’ VO2 max 2.5 to 3 ml/kg/min higher than their lab tests on average, both at the start of training and after 12 weeks.
The researchers still judged it accurate enough to track how fitness changes over time, because its average error stayed under 10%.
My own watch missed in the other direction, and by much more.
Why Is Your Garmin VO2 Max Going Down?
Your Garmin estimates VO2 max by comparing how fast you run with how high your heart rate climbs to hold that pace.
So, anything that raises your heart rate at the same pace, or slows your pace at the same heart rate, lowers the number.
Do Hot Runs Lower Your Garmin VO2 Max?
Heat raises your heart rate at the same pace, so the watch reads a hot run as a less fit version of you.
Researchers have measured how big that rise is.
In a 2005 study, cyclists holding a steady, moderate effort in 95°F (35°C) saw their heart rate climb 12%, from 151 to 169 beats per minute, between minute 15 and minute 45 of the ride.
Their workload stayed the same the whole time.
That means a few weeks of summer running can pull your estimate down even though your training hasn’t changed.
Do Hills, Trails and Wind Lower It?
Running uphill, on technical trails or into a headwind slows your pace while your heart rate stays at the same level.
Your watch sees a slower pace for the same heart rate and lowers its estimate.
When your Garmin VO2 max drops after a week of hot, hilly or trail runs, check where and when you ran before you change your training.
Does a Higher VO2 Max Mean Faster Race Times?
A higher VO2 max doesn’t equal faster performances, and my own lab test proved it.
In college, I took part in a research study in our kinesiology program that included a lab VO2 max test.
My result was 84.5 ml/kg/min.
After my test, the researcher told his colleagues to remember this day because they would never see a higher VO2 max.
My teammate, who was in the same study, scored 72 ml/kg/min.
He beat me in every single race that year, while I regularly found myself mid-pack.
I could take in more oxygen, but he needed less of it to run at the same speed, and that ability is called running economy.
Running economy is how much oxygen you need to hold a given pace, and a more economical runner can hold a faster pace on the same amount of oxygen.
Researchers have tested how much economy matters in runners of similar fitness.
Research has shown that among runners of similar ability, running economy explained 65% of the differences in 10K race times, while VO2 max had almost no link to how fast they ran.
That means once your fitness is close to the runners around you, how well you use oxygen decides more of your race time than how much oxygen you can take in.
Your race times can also keep improving while your VO2 max stays flat.
A 3-year study of elite runners found the younger athletes kept running faster in competition while their VO2 max stayed essentially unchanged.
So a Garmin number that won’t budge doesn’t mean your training has stopped working.
If you want to work on the efficiency side, start with these ways to improve your running economy.
What Should You Use Instead to Set Your Training Paces?
A recent race result is a better guide to your training paces than any watch estimate, because it measures what you can run on race day.
If you haven’t raced in a while, a 5K time trial on a flat course in cool weather does the same job.
Put that time into a pace calculator and set your workouts from it.
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This handy pace calculator helps you to determine what pace you should be running at in training and at each racing distance based on a recent race result.
Your Garmin VO2 max is still useful as a trend.
Compare it over several weeks, and only across runs done in similar weather on similar routes.
If that trend climbs while your race times improve, your training is working.
Set your training paces from a recent race or time trial, and use the watch only to follow the trend.
Should You Still Do VO2 Max Workouts?
Training at VO2 max intensity still raises your VO2 max and makes your heart pump more blood with each beat, whatever the watch says.
A 2007 trial found that 4 x 4-minute intervals at 90 to 95% of max heart rate, done 3 times a week for 8 weeks, raised VO2 max by 7.2%.
The same amount of work done as easy running or threshold running raised VO2 max by less.
The interval groups also pumped about 10% more blood with each heartbeat by the end of the trial.
That’s why every race distance, from the mile to the marathon, can benefit from training at the pace where you reach your VO2 max.
This guide to VO2 max has specific VO2 max workouts for the 5K, 10K, half marathon and marathon.
What Should You Check When Your Garmin VO2 Max Changes?
Here’s a chart of what usually moves the number and what to do about it.
| What you see | Likely cause | What to do |
|---|---|---|
| A drop after hot or humid runs | Heat raises your heart rate at the same pace | Compare only runs done in similar weather |
| A drop after hills, trails or windy runs | Your pace slows while your heart rate stays the same | Judge the trend on flat road runs |
| A flat number while your race times improve | Running economy can improve without VO2 max changing | Trust your race results |
| Race predictions far from your real times | Individual estimates can miss a lab test by about 10 ml/kg/min | Set paces from a recent race or 5K time trial |
3 Responses
maybe the watch only works for people in the middle of the range
Loved that! So logical, and still i do get influenced by all those numbers. Thanks for sharing 🙂
One interesting aspect of the predictions is how they can become a sort of self fulfilling prophecy. People may internalize the predictions, regardless of the fact they may be off by quite a bit re: a person’s potential. Still, come race day they’ll slavishly check their watch at every split, and hew toward said prediction. I myself still wear the Garmin as I do like having the data at hand, though have reverted to listening to my body more so than checking the watch all the time during runs. I’ve had more fun and, more than once, at the end of a half marathon was surprised to see how good my times were when keeping the watch from getting in the way.