Jonny your experience reminds me of a post I made, back in November I think...long story short, if you do a few outdoor runs with GPS on, the Band will calibrate your stride and speed/pace based on the data provided by the GPS sensor. Here's an early article on the topic:
You need to calibrate your Microsoft Band: Here's how
Here's the catch (aligning with
Cicero's post above): it seems the calibration process locks in to
whatever particular speed you run outside using GPS, so if you vary your speed indoor on the treadmill, it's not able to extrapolate accurately far beyond the calibrated run speed. If you teach it how to run at 7.5mph, it won't estimate 5mph or 10mph well.
Personally, I was pretty amazed that speed estimation worked indoor at all. Sure, I was disappointed that if I sped up to a sprint it didn't keep up with me, but with GPS turned off it's pretty tough to estimate your speed without a foot pod. I guess I'd call it a "feature," not a bug.
By the way, regardless of what the neowin article says, my hunch is that the Band
does not use GPS to calibrate normal walking stride length or steps. I'm 99.9% sure it simply uses the pattern of motion in the accelerometer and gryoscope to count steps, and as far as I can tell, it's generally pretty accurate. Otherwise, I think "GPS calibration" is generally for when you engage the Run mode but GPS is turned off.
-Matt