In-reach mini not tracking mileage correctly?

This is to be expected. There is nothing to calibrate. Like all GPS devices, the Mini calculates distance along the track by adding up the straight-line distances between track points. The Mini records track points strictly according to time. If you have set the unit to "send" track points at 10 minute intervals and to "log" track points at one minute intervals, the Mini's idea of distance traveled will be based on the one-minute interval (the shorter of the send and log intervals).

The amount of inaccuracy that this introduces will depend on the shape of the track, the time between points, and your speed. There will be very little inaccuracy if the track is completely straight. That's never the case in practice. The straight-line distance between two points on a curved track is always less that the actual distance along the curve. More curves, higher speed and longer time between track points will all cause larger discrepancies.

On the Mini, it is possible to log at shorter intervals. But logging at 30 seconds or under will cause the Mini to keep the GPS radio on at all times. This drains the battery quickly. The Mini will display a warning if you set logging to 30 seconds or less.

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Twolpert. thank you very much for your help.  How do I set the unit to send track points at 10 minute intervals and to log track points at one minute intervals.  Thanks again.

From the main screen: OK, down-arrow to Setup, OK, down-arrow to Tracking, OK. From the resulting tracking screen, you can adjust the Send Interval ("sent" track points which go to the web site in more-or-less real time via the Iridium network) and Log Interval (logged track points which stay on the device until you sync).

Note that "sent" track points may incur a charge depending on your plan.

To get the logged track to show up on the web site, you need to sync via the free Earthmate app for your phone or via a wired sync via USB connection to your PC.

Good to hear your update.  I have been having the same issue as i compare my Strava app distance to my Garmin MINI.  i went mountain biking today and my MINI (2 min logs) tracked about 3-4 miles less distance than my Strava track and the state park's map.   What log interval did you end up using?  1 minute?  30 seconds?    Also, how is your battery life at the shorter logging points?  Thanks.