 IROC-Zs with the TPI 350 had the 2.77 rear axle ratio as in the previous year, but the optional RPO G92 Performance Axle package modified the ratio to 3.27 for the TPI 350, and 3.45 for the TPI 305 with manual transmission.RPO G92 also included the aforementioned dual-converter exhaust; 4-wheel disc brakes (RPO J65); engine oil cooler; P245/50ZR16 Goodyear Eagle unidirectional tires; a speedometer; and a tachometer with a 5,500 rpm redline.A total of 1,426 IROC-Z coupes were equipped with the Performance Axle package in 1989.