How does Mobility stat correspond to tiles moved per action?

Solution 1:

Looking at ini files (defaultgamecore.ini, line 1508 and defaultgamedata_soldierskills.ini, line 144), there are also meters and unreal units:
1 tile = 96 unreal units
1 meter = 64 unreal units
So 1 tile = 1.5 meters

So Mobility is in meters per action.

Examples (using Legend stats):

• 12 Mobility = 8 tiles (X-COM without PCS, basic enemies)
• 13 Mobility = 8.6 tiles (X-COM with PCS Speed+1)
• 14 Mobility = 9.3 tiles (Stun Lancer Mark1, Faceless)
• 15 Mobility = 10 tiles (Psi Zombie, Stun Lancer Mark2&3, Chryssalid, Berserker without enrage)
• 16 Mobility = 10.6 tiles (X-COM With PCS Speed+3 and GTS bonus, Sectopod)

Fractional speed adds up for dashes and diagonal movement.