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I'm using the Wahoo ELEMNT BOLT with RPM speed sensor. First few rides with my dad I noticed my measured distance was about 2-3% off from his, I used the standard 2.096m size of my wheel (700x23C). He uses the Polar V650 which measures it with GPS. When I let Strava recalculate the distance is also has this difference.

After this I started using the auto-calibrate feature but this error still holds, not much has changed. I also did a rollout test where my circumference come to be around 2.093m. This difference however only accounts towards a 100m difference in distance. Nowhere near the difference that the GPS units calculate.

Anyone of you have any experience with these kind of sensors and data discrepancy? GPS is quite trustworthy when looking at the total distance so I'm not sure what to trust.

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    I'd say that a 3% difference is pretty trivial. Even two riders with identical bikes and meters might see this, if one rider tends to weave more than the other. – Daniel R Hicks May 26 '20 at 13:27
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It may not have to do with sensors or wheel diameter. My gf and I go on rides together both using Strava via phone app. Every time, without fail, the data will claim she rode about .15 per mile more than I, even though we stayed together during the ride. I've tried it with my Garmin and get similar results. I think GPS technology is just jankier than we perceive it to be. I have looked closely at some of the map data after rides and often, my path on the map looks like a squiggly line when I know I wasn't swerving THAT much. Gps is just not perfectly accurate. If your wheel and your dad's wheel are the same diameter and you get different results, this is probably why.

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    Last summer I recorded rides with OsmAnd. When I reviewed my gpx tracks in RideWithGPS, in some areas it looked as though I weaved back and forth across the road. I think it has to do with how well your device (and its software) deals with GPS signals. The elevation data was even more inaccurate. The elevation inaccuracy I understand to be caused by the fact that 1) GPS just isn't that accurate for elevation/altitude, 2) map terrain databases contain a lot of interpolated data for elevation and 3) bike computers that have barometric altimeter are usually considered to be the most accurate – Gaston May 26 '20 at 14:40
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    I find the speed sensor more accurate than the GPS, using the Garmin Edge I always find GPS has me doing a constant 2.3mph even at standstill. I still tend to use GPS though as it's handy for the map function. You should have a setting to select what type of GPS to use, GPS or GPS & GLONASS or GPS & GALILIOE apparently one is more accurate than the other but again it's possibly region based – Dan K May 26 '20 at 14:51
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    GPS, especially professional-level GPS, can be quite accurate--but consumer level devices, especially small ones attachable to one's handlebars, are limited in the size of the antenna they can contain and, when moving, their view of the sky can be occluded by trees, buildings, and sometimes your own body. When you really need accuracy in speed and distance, a sensor based on actual measured wheel circumference is more accurate. – R. Chung May 26 '20 at 15:53
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    Given the errors @R.Chung describes, compact GPS units (e.g. phones) and the software running on them often implement some sort of averaging/smoothing. This can be good (for removing non-existent wiggles while going straight) but can round off really tight turns (usually most noticeable when running as you're moving fairly quickly but can turn on the spot) – Chris H May 26 '20 at 16:12
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    GPS is just a technology. If people cannot be bothered learning enogth to understanding how it works and its limitations, that does not make it janky, – mattnz May 26 '20 at 19:42

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