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)), 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".
So, I assume B is not a shinobi. Then A recommended 兵糧丸 to B and the food has high nutrition. That's why A is not eating anything in a barbecue. A few amount of intake and Long time activity. So, it does not cost much food and you do not have to eat foods a lot. As a result it's fuel-efficient.
The 兵糧丸 made for "Shinobi" should be having higher nutrition than that of for others, then A says 忍のは燃費がいい → '兵糧丸 made for "Shinobi" has high-nutrition' → "good-fuel-efficiency" in this kind of sense.
Well-trained Shinobi might have better metabolism than others though, I think it is irrelevant to Shinobi's metabolism itself.
兵糧丸 = Field ration/MRE
Wiki states that they were made (in different manner) by various warrior/shinobi clans, and there is a PDF Article linked that states that ones for shinobi was primarily made out of sugar with some spices and rice base. It further gives a comparison on different types of rations, and that they were around 20-30g and contained as much as 50-150kcal each, or as much as a box of caramel, a single riceball, or a banana, as per the article:
兵糧丸も飢渇丸も一個は大体20 ～ 30グラムが持ち運びやすく、おおむね一つが50 ～ 150キロカロリーぐらいです。キャ ラメル一箱やおにぎり一個やバナナ一本に相当するカロリーです。
So the efficiency is referring to the amount of caloric intake A already had.
-- Starfox
