Garmin and Zwift not accurate

Or maybe perhaps they are not the same. I just started using Zwift on a Wahoo Kickr and also have a Garmin 520 plus connected.

Mileage on the two is very different.

Additional question which might directly relate to the situation. My tire size is 29x2.25 but I don’t see a setting even close to that. Can I customize one?

Your Garmin 520 does not know about the virtual elevation changes with in Zwift, so Zwift and the Garmin will hardly ever match.

You can use this link for choosing a tire size: