This question is pretty self-explanatory. Gas seems to only go down while driving manually or automatically, but since fast-traveling also relocates the Regalia, it only makes sense that Noctis and co. are driving the Regalia during this time. I haven't experimented enough with fast-traveling much as of yet, since playing other games while auto-piloting across the world to get lazy AP has been my strategy lately, so I'm not sure if it's possible to run out of gas while fast-traveling.
If this can happen, does the game tell you? Or is it just not possible?
Best Answer
No.
Gas doesn't deplete during a fast travel as far as I've been able to determine, however you can only fast travel to locations visited previously by driving to them.
The game does tell you when you're out of gas but I have not seen anything warn me other than the tank meter, or heard any of my other party notify / warn / whine about gas level.