It's really hard to say for sure. I know that before WW2 most cars were american and only in the 50's and 60's did we get cars from the east and rest of the Europe. Many taxis between 1950-1980 were american as you could get some sort of tax deduction when the car was not privately owned. For that reason most corporate executive cars were also US made. Problems started when the government began to tax luxury accessories in cars. A/C, power everything etc. and you could end up paying double the cars value! That's why, in the end, most American cars are in the same price category as E-class Mercedes, even though they only cost a fraction of that in the US. For some reason, GM and Mopar sold somewhat well in the 90's and early 2000. Maybe because none of the competitors offered as much bells and whistles. You have to rember that most of us were quite happy with manual trans + A/C at that time. You just couldn't buy a Toyota or a Volkswagen with power seats and cruise control back then, so your options were limited.
Anyway, I'm glad we have a lot of these oldtimers still available. Like this 1980 Oldsmobile which originally was a diesel, but now has a 5 litre Chevy engine. You could easily swap the engine/transmission to almost anything from Chevy lineup as long as one of the models were offered as a factory standard. They are also pretty cheap, around 4-6k€. I'm pretty sure the US prices have overtaken us a long time ago in this matter.
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