I think they would probably end up building the gas stations or refueling stations on the ground. I also think it would be illegal to fly over a certain altitude. There would need to be a decent buffer between the lowest altitude for planes and the highest for plaars (i'm calling these flying cars plaars), unreal do you have any suggestions on that?
There would probably be emergency tanks that are required to be full if you get pulled over....err under or soemthing. This way, if you run out of fuel (which you never should anyways) in the air then you can use the emergency fuel to get to a gas station. The reason you should't run out of fuel is because like a spare tire, it isn't something you should run on. Only emergencies.
The plaars would need to go through checks at special regulated stations to make sure they are safe to fly and everything.