The joining of dissimilar materials such as aluminium and steel is of growing importance in modern manufacturing, owing to the demand for lightweight structures with superior mechanical performance. This study investigates the influence of rotational speed on key thermo-mechanical performance measures during dissimilar friction stir lap welding of aluminium alloy-6061 and stainless steel-304. Using finite volume method, numerical simulations were performed to quantify maximum weld interface temperature, maximum weld interface velocity, minimum weld interface viscosity, and tool-workpiece interface torque over a rotational speed range of 200–2200 rpm. Results reveal that maximum weld interface temperature rise steeply up to about 1000 rpm and then plateau due to thermal equilibrium. maximum weld interface velocity increases almost linearly with rotational speed, indicating improved interfacial shear and material mixing. In contrast, minimum weld interface viscosity and tool-workpiece interface torque decrease markedly as rotational speed increases, reflecting enhanced thermal softening and reduced resistance to tool motion. Intermediate rotational speed values (≈ 600–1200 rpm) provide an optimal balance of heat generation, material plasticization and torque, minimising the risk of excessive intermetallic compound growth or welding defects. The findings establish a physics-based framework for selecting process parameters that enhance joint integrity and efficiency in dissimilar friction stir lap welding of AA6061-SS304.