Automatic wheel circumference underreporting (since FW 1.571.2185)

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Christian Renner

• July 27, 2025 08:55

I was surprised after my recent rides that the average speeds were a bit low (ca. 1 km/h less) despite similar average power and routes compared to a couple of weeks ago. I also found that distances were several kilometers shorter than previously. I checked the GPS distances and found that my Karoo (3) has been **underreporting distance by ca. 3-6% on all workouts since July 11th**, when I updated to **firmware 1.571.2185**. Before that, all distances where pretty much spot on compared to GPS distances (error within few hundred meters at 100km distance) *and* compared to distances obtained from planning the routes with rwGPS and Komoot. I observed this behavior on two different bikes / with two different wheel sets. Both wheels use Garmin speed sensors (two sensors, one for each wheel set), and the sensors are configured to auto-calculate wheel circumference. I checked the circumference for one wheel yesterday and it reported 2083mm, which is way too low for a 622mm/28" wheel with 30mm tires (actual 32mm width, so circumferences should be > 2150mm, maybe around 2180mm).

Executive summary:

- Large discrepancy of GPS and sensor-based distances (3-6%)
- using auto-calculation of wheels circumference
- since firmware 1.571.2185

This is a major regression. It affects multiple sensors, and I can definitely track this down to a single date / firmware update.

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  Bhagyashree

  • August 11, 2025 10:05

  Hi Christian, as I can see, there is already an open ticket for this issue, and Bijit is assisting you with it. He will help you resolve this issue.

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  Christian Renner

  • August 11, 2025 10:17

  Hi Bhagyashree, thanks for your heads-up on this. We've been going through this, and I have done a lot of testing to make the issue clear. The main findings are as follows, and I think that multiple users are affected (but they might not know or realize):

  - All works fine, if the speed sensors are connected using ANT+. Auto-calibration works, and distances are very precise.

  - If using BLE to connect speed sensors (I tested four magnet-less sensors attached to the hub (front and rear) from Wahoo and Garmin), the reported distance is too low (ca. 2%), auto-calibration produces too small circumference (another 2%), and the speed trace is way more noisy (spikes that indicate lost packets).

  I really think this should be investigates and fixed.

   

   

   

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  Bhagyashree

  • August 20, 2025 08:16

  Hi Christian, Thanks for sharing your detailed findings. As a workaround, you can manually enter the tire circumference in the sensor settings to improve accuracy. There’s also a software update coming soon, so please check again after updating.

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  Christian Renner

  • August 24, 2025 16:39

  Thanks for the heads-up, and great to hear there will be a firmware update. As stated above, the issue is essentially with BLE sensor connection. With ANT+, accuracy is much better (though still a bit underreporting, but typically within 1%).

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