A notable feature of bacterial chemotaxis is adaptation : after the application of a step stimulus, the bacterial running time relaxes to its pre-stimulus level. The response to the amino acid aspartate is precisely adapted whilst the response to serine is not, in spite of the same pathway processing the signals preferentially sensed by the two receptors Tar and Tsr, respectively. While the chemotaxis pathway in E. coli is well characterized, the role of adaptation, its functional significance and the ecological conditions where chemo- taxis is selected, are largely unknown. Here, we assay the role of adaptation by experimentally investigating E. coli speed races in gradients of aspartate, serine and combinations thereof. By using microfluidic techniques, we engineer controlled gradients and demonstrate that bacterial fronts progress faster in gradients of serine than aspartate. The effect is observed over an extended range of concentrations and is not due to differences in running velocities. We then show that adding a constant background of serine to gradients of aspartate breaks the adaptation to aspartate, which results in a sped-up progression of the fronts. Experimental observations are theoretically related to the coupling between sensing and running that controls the chemotactic up-gradient drift.