So I got back to running today and took my Garmin forerunner out.
Now I run on treadmill. At the end of my run my distance was recorded as 3.8k.
According to my Garmin my distance was 4.6k.
Big difference if I’m aiming for a 5k.
Can anyone shed some light on which I should go off?
The treadmill is going to be more accurate. The Garmin is relying on GPS which has a margin of error. 👍
Saying this, I've just analyzed this morning's run in an app called "Footpath" which allows you to trace your exact route (including any footpaths and things). It said my run length was 2.14 km, Garmin gave it as 2.04 km. So the Garmin underestimated on this occasion.
Now I'm not entirely sure what to say about your question! 😂
GPS ??? On a treadmill ??? How does that work Hidden ?
The watch is using GPS not the treadmill?
Nope. Think about it . What does GPS do? What does GPS stand for?
LOL, I'm not 100% sure but it won't work indoors if it can't get the satellite signal. Am I right?
EDIT: And yet I've just tried it sitting in my living room. It took a moment more but I still got the GPS Ready notice.
EDIT2: And then I just realised. You're not moving position on a treadmill. Oh dear, now I feel like a total idiot! 🤣🤣🤣🤣
GPS = Global Positioning System - a satellite system that calculates your position on the Earth.
If you're running on a treadmill - ANY treadmill - then your position on the Earth is going to change is it ? You're in the same spot for the entire duration of the run.
YAY! See my reply to Chris below as to how these watches measure distance on a treadmill 
I don’t know how it works indoors but outdoor running the Garmin GPS watches are very accurate. I’d go off the data on the treadmill.
Damien
Hi Chris,
Assuming you have the GPS switched off (!), the distance your Garmin measures on a treadmill is estimated by using the in-built accelerometer which monitors the movement of your arms. It's from that that it estimates your steps-per-minute and calculates your stride length and therefore your distance.
The treadmill's recorded distance will be a LOT more accurate than the watches estimation.
My money is on the treadmill being more accurate, but neither being 100%.
When you finish a treadmill run (you have to select this sport mode in your Garmin) instead of saving, you can scroll down and you find a calibrate and save option. You can adjust your Garmin slightly to match the pace in the treadmill and it will be better for the next run (but it will never be too accurate, as explained before, in the treadmill the calculation of your watch is based on your wrist movement and your height, and assumes and average relationship of those parameters and speed...)
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