Aside from a new grille, the 1975 models were virtually identical to the 1974s, except that California and certain high-altitude models were equipped with catalytic converters and so required unleaded gasoline. All 1975 models were required to pass a roof crush test and to meet this stringent requirement, and additional reinforcements were added to all Dart 2-door hardtops. Heavy gauge steel in the windshield pillar area had been incorporated into the windshield, pillar and roof design. Darts were also equipped with an improved energy absorbing steering column which used multiple slots in the column jacket to replace prior used convoluted mesh design. At impact, force applied to the steering wheel curled the column jacket back over a mandrel mounted on the floor. Federal Motor Safety Standards briefly required that the front seat belts include a starter interlock system that prevented the engine from starting unless the front seat outboard occupant and the driver fastened their belts.
The 198 Slant Six engine was discontinued and the 225 became standard equipment on all models. As in 1974, the 360 V8 was limited to the Dart Sport 360 model. A 4-speed manual transmission was offered with the 6-cylinder engine for the first time in the North American market since 1965, and with a new 30% overdrive 4th gear ratio. It was Chrysler's first application of overdrive since 1959. The final drive ratio in fourth gear was 2.36:1 on the Slant Six cars equipped with 3.23:1 rear axle, and 2.15:1 on the V8s equipped with 2.94:1 rear axle. The result was less engine noise and wear and greater fuel economy.
Also for 1975, heaters had 14% more heating capacity than in the previous year's models. The added capacity was the result of adding a third speed to the blower system, which provided 20% greater air flow at maximum setting. The electrically heated backglass defogger grid timer cycle was doubled to 10 minutes. Additionally, sound insulation was improved.
Source: Wikipedia, 2012