Austempered Ductile Iron (ADI) samples were heat treated to produce materials with tensile strengths in the range of 100 ksi to 170 ksi. Microalloyed steels were also produced with equivalent tensile and yield strength levels. These steels were evaluated for mechanical properties in terms of tensile and yield strength, ductility, impact toughness, fracture toughness and fatigue strength. Machinability was extensively evaluated through tests of drilling, turning and plunge machining. This paper reports on this comprehensive comparative evaluation of these two important classes of materials for use in the automotive industry.