Yeah, cars do not have particularly high requirements in terms of energy density. In a pinch people have been able to substitute petrol with fuel as low density

Yeah, cars do not have particularly high requirements in terms of energy density. In a pinch people have been able to substitute petrol with fuel as low density as wood: https://en.wikipedia.org/wiki/Wood_gas_generator https://en.wikipedia.org/wiki/Wood_gas_generator But that's just the driving around use case, others are very far out of reach.