In 1975 horsepower was on a steep decline in the Firebird line but sales were on the way up with 84,063 sold!  The Super Duty engines were gone forever and the catalytic converter was now required on all models.  The Firebird received a new wrap-around rear window treatment and the turn signals moved to the upper grille.  The Trans Am came only with a 400 cubic inch engine at first but at mid-year a 455, called the 455 HO but not related to previous 455 HO engines, was reintroduced due to public demand.  RTS or Radial tuned Suspension was now standard on all Firebirds.  Even with reduced horsepower the Trans Am was the last musclecar with big block engines and a record 27,274 were sold, almost triple that of 1974 with 10,255 sold.  Popularity of the Firebird models would only increase. 

Marketing of the Firebird took a drastic turn in 1975 with emphasis on fuel economy:

"Expect better overall economy.  You could buy a 1975 Firebird just for the European-inspired styling.  Or for its comfortable ride.  Or its sports car handling.

This year you can enjoy Firebird's famous comfort along with several improvements...and just one of them is the expectation of getting better overall fuel economy.

1975 Firebird's got new ideas.  Every Firebird model now offers a High Energy Ignition system.  Its solid state circuitry replaces moving parts, and helps supply more power to the spark plugs."