When deciding upon which fuel source used to power a vechicle, engineers often use the heat released in combustion of differning fuels to make their decision. Would you use the heat released per mole of a hydrocarbon or the heat released per gram of a hydrocarbon to make a decision. State why from an engineering standpoint.

Question

When deciding upon which fuel source used to power a vechicle, engineers often use the heat released in combustion of differning fuels to make their decision. Would you use the heat released per mole of a hydrocarbon or the heat released per gram of a hydrocarbon to make a decision. State why from an engineering standpoint. I understand that the fuel has to be carried in a fixed mass tank by the vehicle but I'm not really sure how to answer this question.