[tropo]

Car manufacturers would be really effective, though far from efficient. Under our current conditions, it makes sense to use car manufacturers.

Car manufacturers rework and reuse things all the time. There are always new models coming out, mainly to look different from previous models. Lots of parts get repurposed, being used for very different vehicles. Lots of parts get redesigned each year, often for small improvements.

Car manufacturers also have great small-volume production for engineering prototypes. It isn't as cheap as normal production, but it can be put to use immediately.