The claim regarding the Mustang is a bit of a misleading statement. The Fieros initial year production run began in July 1983 and concluded late in 1984. The exact date of change from 84’s to 85’s isn’t in any literature I have, but it is likely late November or early December for the changeover. All Fiero’s produced in that period were considered ‘84’s. During that time about 136,840 Fieros were produced. Assuming the changeover was November 84, that is 17 months of production.
The Mustang was introduced in April 1964, production beginning in March of that year. All Mustangs produced from that initial launch until the end of 1965 were considered 1965’s. However, production of Mustangs in 1964 was 121,538 cars. If we make the same assumption of late 1964 that is 9 months of production. In reality there were rolling production line changes which do distinguish early cars from later cars that began in August and went for several months. Still, all Mustangs produced from March 1964 through the end of 1965 were considered 1965s. In total there were 559,000 Mustangs built in that time. More Mustangs built in that time than the entire Fiero production run of 5 years.