In this paper, we propose an adaptive technique exploiting transmission mode switching between multiple input multiple output (MIMO) and single input multiple output (SIMO) with antenna selection to conserve mobile terminals' energy. We focus on saving uplink RF transmission energy in cellular systems supporting dynamic best effort (file transfer) traffic loads. The key idea is to judiciously slow down file transfer rates when a base station is underutilized. Due to the DC power components associated with the multiplicity of transmission chains, MIMO may have higher power consumption than SIMO. Thus, considering a desired user perceived target throughput as well as energy-efficiency, we propose an algorithm for mode switching (MIMO/SIMO) and rate selection. Extensive flow-level simulations under dynamic loads and Rayleigh fading channels confirm that the proposed technique can save more than 50 % of the mobile terminals' transmission energy and enable an effective tradeoff between performance and energy conservation.