Complex sentence: The decade began with 25 million registered automobiles on the road, most of which pre-dated World War II and were in poor repair; no automobiles or parts were produced during the war owing to rationing and restrictions.
Split into simpler sentences: The decade began with 25 million registered automobiles on the road, most of which pre-dated World War II and were in poor repair. No automobiles or parts were produced during the war due to rationing and restrictions.