Complex sentence: Sales ran from 1954-79 by which time more than 20,000 had been built making it Leyland's most sucessful bus; by 1985 approximately 17,000 Atlanteans had been built, Leopard sales terminated in 1983 after deliveries totalled over 12,000.
Split into simpler sentences: Sales ran from 1954-79 by which time more than 20,000 had been built making it Leyland's most sucessful bus. In comparison, by 1985 approximately 17,000 Leyland Atlanteans had been built, Leyland Leopard sales terminated in 1983 after deliveries totalled over 12,000.