Forgot to stop your Garmin: how to remove the drive home

You finish the ride, put the bike on the rack, and remember the watch two towns later. Now the activity has a 90 km/h top speed, an extra 20 km, and an average that means nothing. The fix is to cut the part you didn't do under your own power out of the file and upload the clean version.

When Strava's crop is enough

If the extra part is only at the end and you only care about Strava, its built-in crop handles it: open the activity, choose Crop from the three-dot menu, and drag the end handle back to where you stopped.

Edit the file itself when:

The extra part is in the middle. You got a lift partway, took a train between two sections, or left the watch running through a long lunch. Strava's crop only trims the ends.
You want the original right everywhere. Garmin Connect, training load, and any other app the file goes to all read the same file, so a fixed file fixes all of them.
You want to keep the file. A cleaned-up .fit is the version worth archiving.

Step 1: get the file

From Garmin Connect: open the activity, click the gear icon, choose Export Original. You get a .zip; the editor opens it directly.
From Strava: open the activity, three-dot menu, Export Original.
From the device: plug it in and look in Garmin/Activity for the .fit file with that day's date.

Step 2: cut the car section

Open the file in FIT File Editor. Drag it onto the page or press browse.
Look at the speed line on the timeline. The drive shows up as a steady block far above your riding or running speed, and the map shows it following the road home.
Drag across that stretch on the timeline. For the end of an activity, drag from where you stopped to the very end.
Press Cut. The distance, time, averages, and top speed are recalculated from what is left, and the laps around the cut are adjusted.
Press Export and save the new file.

The file is read and written in your browser; nothing is uploaded.

Step 3: replace the activity

Delete the original activity first, then upload the edited file. If you upload first, Strava and Garmin Connect both see two activities at the same time and reject the new one as a duplicate. The duplicate activity guide covers the order that works on each platform.

Worth knowing

Moving time stays honest. A cut in the middle becomes a pause, so elapsed time still covers the whole day while moving time covers only what you did.
Top speed and distance are your own again. Once the 90 km/h stretch is gone, the activity's top speed, distance, and averages stop including the car.
Same steps for a train, a ferry, or a chairlift. Anything you rode rather than ran, walked, or pedaled cuts out the same way.

For the tool on its own, see Crop a FIT file.