Fuel

Feeling drives; motivation; whatever has ‘fuelled one’s drive’. Petrol/gas is also a resource, something you know you can call upon to achieve something or ‘get somewhere’. So having an empty tank would suggest you have no such resources of energy, or motivation or ‘drive’ to achieve what you desire or need to do. It can sometimes indicate exhaustion and poor health. So it might indicate a change of diet or life style – getting more rest?

There is an interesting complexity here as you need resources – money – to get the resource of fuel. See Petrol