The increasing complexity of system-on-chip (SOC) integrated circuits has spurred the development of versatile automatic test equipment (ATE) that can simultaneously drive different channels at different data rates. Examples of such ATEs include the Agilent 93000 series tester based on port scalability and the test processor-per-pin architecture, and the Tiger system from Teradyne. The number of tester channels with high data rates may be constrained in practice however due to ATE resource limitations, the power rating of the SOC, and scan frequency limits for the embedded cores. Therefore, we formulate the following optimization problem: given two available data rates for the tester channels, an SOC-level test access mechanism (TAM) width W, V (V < W) channels that can transport test data at the higher data rate, determine an SOC TAM architecture that minimizes the testing time. We present an efficient heuristic algorithm for TAM optimization that exploits port scalability of ATEs to reduce SOC testing time and test cost. We present experimental results on dual-speed TAM optimization for the ITC?2002 SOC test benchmarks.