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In this paper, we propose an improvement of truncated ARQ protocol throughput through the optimization of the distribution of the average energy cost per packet (or, equivalently, per symbol) among all packet transmission attempts. We consider exclusively BPSK-modulated ARQ transmission over an AWGN channel, with and without packet combining using the selective repeat as a transmission protocol. Our optimisation approach is based on a specific choice of the symbol energy of packets during transmission attempts (first transmission and potential subsequent retransmissions), while keeping the total energy cost per transmitted packet (or, equivalently, per symbol fixed. For a given Signal-to-Noise Ratio, the optimum distribution of the total energy cost among packet transmission attempts is obtained using a dichotomy search. Numerical results show that the energy distribution is almost surely unevenly distributed among packet transmission attempts. More specifically, for low SNR, almost most of the total energy is allocated to the first transmission while for large SNR, the allocated energy keep increasing with unsuccessful retransmission attempts. Numerical results also reveal a significant improvement, for medium SNR, of both throughput and packet erasure rate, relatively to conventional ARQ.