tran-zam.com
1975 Facts and Interesting Notes
The Wrap Around Rear Window Introduced
For 1975 the most visible difference in the body styling was in the growth of the rear window to wrap around to the fender top.  This increased outward visibility for the driver by 10% and looked more natural with the car's body lines.  This style window was planned originally for the 1970 model but problems with the glue squeezing out during installation caused its delay.
Updates to the Front End Styling
For 1975 the turn signals moved from the lower valance to the upper grille and a new grille pattern was used featuring horizontal grilles instead of the vertical grille of 1974. 
The Catalytic Converter
The catalytic converter was now standard on all Firebirds in keeping with government emissions laws.  Due to the expense of catalytic converters all Firebirds now had single exhausts, though performance models did have dual mufflers and tail pipes giving the appearance of dual exhausts. 
The 455 HO
Originally the Trans Am was offered only with one 400 cid engine at 185hp, down a full 40hp from the 225hp 400 in 1974. On April 25th, 1975 Pontiac announced the return of the 455.  The 455 HO was rated at 200hp and was taken directly from the divisions large car lines.  The engine was not a performance designed engine with virtually no heavy duty parts but it did receive a 4bbl carburetor and a 3.23 rear axle ratio. The HO label would take much criticism since the engine was in no way related to the 455 HO of the 1971 to 1972 years.  But remember that the original Firebird HO was a 350 model in 1967.  The 455 HO equipped Trans Ams are highly desirable now since only 857 were made.  All 455 HO's came with 4 speeds and were not available in California
Formula Engines
The Formula could now only be had with the 350 or 400 engine options. The 455 would never again be offered in the Formula model.
Esprit Engines
The 6 cylinder engine was now standard in Esprit, replacing the 350 of previous years.  Fuel economy was now the main emphasis on all GM products.
Radial Tires
Radial tires were now standard on all Firebirds.
The Rear Spoiler Bird Decal
The Rear Spoiler Bird Decal was now offered in Charcoal in addition to the Blue, Green, and Orange birds of 1974.  The Black decal and the White decal were discontinued for 1975.
Cordova Top Available on Trans Am
This would be the second and last year that a Trans Am was offered with a vinyl top and the only year that the vinyl top would wrap around the enlarged rear window.  In the following years the Firebird Vinyl Top would begin above the rear of the side window moving forward.  The 1875 certainly was a unique look. 

Interestingly I always thought that the 1979 movie Superman showed a red Trans Am with a white vinyl top during the earthquake scene on the Golden Gate Bridge.  In actuality what you see is the front of a red 1975 Trans Am screeching to a stop (no top was visible) and then the car crashes into a pile up of other cars.  However, in the crash the car is not the red '75 Trans Am but a red 1967 Corvette with a white painted top and a TA screaming chicken on the hood!  I guess they did not want to wreck the TA.
Front Disc Brakes
Front Disk Brakes were now standard on all Firebird models.
Speedometer in Kilometers
The speedometer was changed this year to display kilometers per hour  in addition to miles per hour.  Metric measurements were being pushed by the government in the 1970's but never really took hold in the US.  The speedometer now read only to 100mph versus the 160mph speedos of 1970 to 1974.
Fuel Economy Guages
Option UR1/704, a Fuel Economy Vacuum Gauge was offered for the first time on Base Firebird, Esprit, and Formula.  The gauge would display a needle that would point either towards power or economy.   The driver was encouraged to drive the car economically.