Ergonomics in tanks is an important factor, one that I touched on previously in some detail. It turns out that despite certain prevalent stereotypes, the ergonomics of various tank schools are more complicated than many people believe. Having already applied Soviet ergonomics standards to a German design, let us hop across the pond and see what the Americans thought about the topic.
Thankfully, this time, the Fort Knox Medical Research Laboratory report "Adequate Head Room in Tanks" provides us proper measurements, which makes my job a lot easier. The adequate head room for a sitting crewman, excluding the upper and lower 5% of men, is stated as 34" to 38.25", or 86-97 cm. The Soviet "dimensions of an average man" define the same measurement to be 90 cm (35.4"), which falls pretty well in the middle of that range. The American tank helmet adds a whole 1.5" to the height of a tanker by his crash helmet, a thickness that is found to be excessive. At the time of the study (November 27th, 1942), a thinner helmet was already being tested.
Let's see how well average people could fit into American tanks.
Crewmen would have to be incredibly small to fit into the M5 Light Tank. A loader's lowered seat gives only 33.5" of height, excluding almost all men (99.82%) from serving comfortably in this position. The tank commander does a little better, but still only accommodating the shortest 5.53%. Compared to the turret crew, the hull crew lives in luxury, with 35.25" of head room, which is good enough for 26.41% of men according to the study. The study is very critical of the ergonomics of the M5: "It has been determined that the tank can be driven satisfactorily if the maximum required head room can be made available. A major change in seat design is required to effect this change."
The Soviets never ordered the M5 Light Tank, but experienced the same ergonomics issues on the M3A1, among others.
Next, the Shermans. As expected, the medium M4A2 provides significantly more space for its crewmen than the light tank. The loader enjoys more room than anyone in the Stuart, which meets the Soviet requirement for an average man and allows for 42.30% of Americans sampled to serve in that position. Every other position in the Sherman is even roomier than that, and you can figure out who can fit into it on your own with the data from the report.
To finish off, let's go back to that German example. We already found it deficient by Soviet standards, but how many Americans would be fit to drive their tank?
The height of the driver's compartment is only 80 cm (31.5"). This is not only less than any position of the Stuart (a light tank!), but out of the 541 men sampled in the 1st Battalion, Armored Force Replacement Training center, not a single tanker would be capable of comfortably occupying this position.
The report also includes a diagram of the driver's seat in the Stuart, so let's see how well it does compared to the cramped Soviet driver measurements.
As we already knew, the driver's compartment isn't quite tall enough to fit an average Soviet driver. The leg room seems a bit tight as well compared to the Soviet standard (instead of the 58 required cm of leg room there is only 40), but far from the disaster we see in the German design.