Vehicle refueling
All vehicles consume fuel during use. Even with the engine running and idling, the tank empties a bit.
There are currently two ways to refuel your vehicle.
Refueling by canister
To refuel your vehicle by using a canister you first have to buy a canister filled with fuel at the general store or produce it by your own. You also could use the auction house.
When the canister is in your inventory just look at the vehicle, hold the interaction key and choose the option "Refuel (canister)".