Abstract Background Most amino acids are encoded by multiple synonymous codons. However synonymous codons are not used equally and this biased codon use varies between different organisms. It has previously been shown that both selection acting to increase codon translational efficiency and selection acting to decrease codon biosynthetic cost contribute to differences in codon bias. However, it is unknown how these two factors interact or how they affect molecular sequence evolution. Results Through analysis of 1,320 bacterial genomes we show that bacterial genes are subject to multi-objective selection-driven optimisation of codon use. Here, selection acts to simultaneously decrease transcript biosynthetic cost and increase transcript translational efficiency, with highly expressed genes under the greatest selection. This optimisation is not simply a consequence of the more translationally efficient codons being less expensive to synthesise. Instead, we show that tRNA gene copy number alters the cost-efficiency trade-off of synonymous codons such that for many species such that selection acting on transcript biosynthetic cost and translational efficiency act in opposition. Finally, we show that genes highly optimised to reduce cost and increase efficiency show reduced rates of synonymous and non-synonymous mutation. Conclusions This analysis provides a simple mechanistic explanation for variation in evolutionary rate between genes that depends on selection-driven cost-efficiency optimisation of the transcript. These findings reveal how optimisation of resource allocation to mRNA synthesis is a critical factor that determines both the evolution and composition of genes.