Thanks for the responses, gents. I'm replying-to-all to save time and coalesce my response into a couple of key thoughts that apply to all.
There are 2 fundamental flaws in the proposed concept of US mechanical superiority. The first is that other countries had a similar proportion of working-age men engaged in agriculture compared to more technical industries (e.g. Shortround's comment about "many of those military age men (18-40) even from rural areas had at least some exposure to cars, or tractors or engines of some sort."). The second, and far more significant, is the creation of a false dichotomy where car ownership/awareness of the internal combustion engine is directly correlated with technical awareness/proficiency.
In the 1930s, the UK had a greater proportion of men engaged in industry rather than agriculture compared to the US (figures for 1940 agriculture/industry: US - 18.5/23.4; UK 10%/36%) (Sources:
US ,
UK). Almost all that industrial output required mechanics of some sort or other, whether maintaining pit engines, factory steam plants, machinery or whatever (e.g. one of my cousins who died in WW1 worked in a brewery but his job was maintaining a steam engine).
Even in rural areas, lower tractor ownership did not equate to a lack of technical expertise. The internal combustion engine was not the only means of providing automotive power, particularly in rural areas. In the UK, a great many farms were still using steam traction engines well into the 1930s. Many of these engines were decades old and required replacement parts to be hand-made/fettled by the owner or the local blacksmith (maybe that's where the workforce came from to hand-make all those Merlin engines?
).
I come from a working-class industrial town and there were literally hundreds of mechanical firms supporting the town; everything from metal forging to glassmaking to toolmaking and machining. Almost the entire town built, operated or maintained mechanical systems of one sort or another. To suggest that, somehow, these men were technically bereft because they couldn't drive and hadn't operated an internal combustion engine is ludicrous. To cite a personal example, my family only got its first car in the early 1950s. However, my three uncles collectively built the car from multiple boxes of bits. My family was dirt-poor working class but my uncles were all sufficiently technically-minded to build a car from components, and two of them had been driving for well over a decade when they made the car...but they'd never owned a car before.
My fundamental gripe is the over-emphasis of car ownership on the penetration of technology within society. America had certain factors which promoted the internal combustion engine. However, the absence of those factors did not make other populations any less technically savvy/aware, percentage-wise across the workforce.