A Plymouth table directly comparable to the fleet-only Custom 500 table does not exist, so I constructed one by adding the PK, PT and PM production numbers, excluding Canada-only PM23:
1975: 25,432
1976: 20,811
1977: 27,273
The tendencies for 1975-to-1976 are similar: a decline (Ford: -7.52%; Plymouth -18.17%) that far exceeds the decline in the low-priced fullsize market as a whole (-1.35%). It is true that also the higher trim levels not included in the Ford and Plymouth "fleet tables" attracted some fleet sales, but this cannot be quantified and would apply to both makes.
The uptick in 1977 Plymouth fleet production numbers has already been explained:
All Plymouth fleet sales taken together, its share in the fleet market was relatively stable, going from 4% in 1974 to 3% for the 1975-1977 period. So changes would be primarily in the mix of Plymouth models sold to fleets.
1975: 25,432
1976: 20,811
1977: 27,273
The tendencies for 1975-to-1976 are similar: a decline (Ford: -7.52%; Plymouth -18.17%) that far exceeds the decline in the low-priced fullsize market as a whole (-1.35%). It is true that also the higher trim levels not included in the Ford and Plymouth "fleet tables" attracted some fleet sales, but this cannot be quantified and would apply to both makes.
The uptick in 1977 Plymouth fleet production numbers has already been explained:
The main increase in fleet sales in 1977 was that Chrysler announced the end of production for the 1978 model year.
All Plymouth fleet sales taken together, its share in the fleet market was relatively stable, going from 4% in 1974 to 3% for the 1975-1977 period. So changes would be primarily in the mix of Plymouth models sold to fleets.