If you want the circumference make a small mark on the edge of the tyre (on the outside) and a mark on the floor, and align the two. Roll the bike forward until the mark on the tyre is against the floor again. Measure the distance between the two marks on the floor.

The speedo has no way of knowing what tyre you've got on the wheel so all it knows is that each time the magnet goes past the sensor you've moved as far forward as the circumference of the wheel. In this context "wheel" means "rim plus tyre".

You don't really need to do a 10k trip to check it against the GPS, to find out if you're about right do a 1k trip and if they look like they align feel free to do a longer trip if you feel the urge.