What does 燃費がいい mean here?


I was reading a manga where A, a shinobi, is in a barbecue but is not eating
anything. B finds it strange and asks what's up. Then, the talk follows:

> A: さっき、兵糧丸食った  
>  B: ひょうろうがん・・・  
>  A: 忍のは燃費がいい  
>  B: どれ

I don't understand what A's last phrase means. Does he mean the shinobi have
an effective/convenient metabolism and hence don't need to eat much? Or does
he mean that the hyourougan takes time to be metabolized?




燃費 is basically a technical term to describe the efficiency of engines or
gasoline. A 燃費がいいエンジン can work longer with the same amount of gasoline.
燃費がいいガソリン can make an engine work longer for the same price (i.e., cost
efficiency) or with the same amount (i.e., per-liter efficiency).

Nothing surprising happens when this word is used figuratively to describe a
person or food. A 燃費がいい人 refers to someone who does not have to eat much to
live. This sometimes implies they can easily get obese. 燃費がいい食べ物 can refer to
either the cost-efficiency or the per-gram efficiency, depending on the
context.

In your context, 忍の ("shinobi's ones") refers to the 兵糧丸 made or used by
shinobi. Note that [this の is a pronoun (`'s
one(s)`)](https://japanese.stackexchange.com/q/9379/5010), so the sentence is
not about shinobi but about the food itself. And it's obviously about the
"per-gram" efficiency of the food in this context. All military rations are
supposed to be lightweight and nutrient-rich, but A is saying that the one
made by shinobi is especially good in this regard.

If the sentence were 忍は燃費がいい (without の), it would mean "Shinobi don't have to
eat much".




燃費 is basically a technical term to describe the efficiency of engines or
gasoline. A 燃費がいいエンジン can work longer with the same amount of gasoline.
燃費がいいガソリン can make an engine work longer for the same price (i.e., cost
efficiency) or with the same amount (i.e., per-liter efficiency).

Nothing surprising happens when this word is used figuratively to describe a
person or food. A 燃費がいい人 refers to someone who does not have to eat much to
live. This sometimes implies they can easily get obese. 燃費がいい食べ物 can refer to
either the cost-efficiency or the per-gram efficiency, depending on the
context.

In your context, 忍の ("shinobi's ones") refers to the 兵糧丸 made or used by
shinobi. Note that [this の is a pronoun (`'s
one(s)`)](https://japanese.stackexchange.com/q/9379/5010), so the sentence is
not about shinobi but about the food itself. And it's obviously about the
"per-gram" efficiency of the food in this context. All military rations are
supposed to be lightweight and nutrient-rich, but A is saying that the one
made by shinobi is especially good in this regard.

If the sentence were 忍は燃費がいい (without の), it would mean "Shinobi don't have to
eat much".

