1970 RM23 numbers which include the Superbird (I doubt Superbird varied significantly from this data) 15" wasn't that popular

tires:
T86 F70-14 "White Streak" STANDARD ~ 41.1%
T87 F70-14 w/RWL ~ 50.9%
U84 F60-15 w/RWL ~ 7.8%

wheels:
hub cap STANDARD ~ 12.6% (14" & 15")
W11 deluxe wheel cover ~ 14.0%
W15 wire wheel cover ~ 1.9%
W21 RALLYE Wheels ~ 36.0% (14" & 15")
W23 ROAD Wheels ~ 35.5%

ROAD Wheels & F70-14 w/RWL POLYGLAS tires is the most plentiful installed choices