Measuring distance - alternatives?

(Posted first in AppleFitnessPlus group - but was directed here for possibly more answers!)

So i realise that the discrepancy between my treadmill and apple watch is something i have to live with!! I have calibrated and recalibrated the watch but still one or the other is way out (varies but i have had almost a mile out in a 30 minute session in favour of the watch) - and when calorie counting and therefore getting false benefit of calories burned (possibly) it is frustrating. (I cannot for the life of me understand if Peloton integrate with other treadmill makers - when their business is selling treadmills - why apple doesn’t also do this instead of this exclusive gymkit rubbish. Anyway - seeing as i can feed my HRM from a strap or other third party measuring HRM - is there an equivalent distance tracker that i can use that might be more accurate (a foot thingy for example?). Or alternatively is there a bluetooth gymkit add on for a treadmill that can measure distance? Something that measures the belt as it passes etc. thanks if anyone has any ideas.