When I was looking at the efficiency of meat I looked some at the efficiency of alternatives. I noticed that seitan is made by keeping just the protein component of wheat and throwing away the rest, and wrote:

While flour is 10-15% gluten, seitan uses high-gluten flour, and so about 14% of the original wheat becomes seitan. This is about 7 calories [of] wheat per calorie [of] seitan. This is worse than the 2.2 calories [of] corn per calorie [of] beef we got above.

Aside from being confusing, this has a key mistake. The problem is that neither meat nor meat substitutes are entirely protein. I compared beef at around 25% protein to pure gluten at 100% protein. In other words if you want to get the same number of grams of protein from seitan as you get from an amount of beef you only need to eat 1/4th as much. We should redo the calculation to go from pounds of raw grain to grams of protein [1] to get a fair comparison. [2]

To get 100 grams of protein from wheat (as gluten) you would need 1.5 pounds of grain while to get it from corn (via beef) you would need 3 pounds. [3] To make veggie meat with the same protein density as beef, you'd need ~2 pounds of wheat per pound of veggie meat, compared to ~4 pounds of corn per pound of beef. Seitan isn't as inefficient as I had thought.

(I think other kinds of veggie meat like tofu, tempe, and TVP are more efficient than seitan by this metric. Though they are also concentrations and I haven't looked at them in detail.)

[1] It's weird that we use British units for typical measurements but metric ones for nutritional quantities.

[2] This also conflates 'wheat' with 'flour', but this turns out not to be a problem because 'wheat' generally means 'wheat berries' and 'flour', whole-wheat at least, is just ground wheat berries. So one pound of wheat yields one pound of flour.

[3] 100g is 0.22 lbs. 14% of wheat is protein, so 0.22lbs of wheat protein requires 1.57 lbs of wheat. 25% of beef is protein and a pound of beef requires 3.7 pounds of corn, so 0.22lbs of beef protein requires 3.25 lbs of corn.