Mixtures of seven n-carboxylic acids (n-CA), ranging from acetic acid to n-octanoic acid, that can be gained from the microbial conversion of waste biomass, were converted into liquid drop-in fuel via Kolbe electrolysis. We demonstrate a strong, sigmoidal correlation between the total n-CA concentration and the Coulombic efficiency for fuel-like liquid products (CE_fuel). Crucially, only above an apparent threshold concentration of 0.1 mol L⁻¹ n-CA, Kolbe electrolysis takes place, whereas at lower concentrations the oxygen evolution reaction (OER) dominates. Optimal performance was achieved at the highest concentration tested, 1.5 mol L⁻¹, yielding a maximum CE_fuel of 63.8 ± 1.4%, whereas for n-CA to 0.1 mol L⁻¹ and 0.5 mol L⁻¹, the chain shortening reaction (CSR) strongly affects CE_fuel. Furthermore, the study provides valuable insights into reaction selectivity, demonstrating that longer-chain n-CA react preferentially compared to shorter-chain n-CA, a phenomenon that is attributed to their higher hydrophobicity and hence accumulation in the hydrophobic layer formed on the electrode. Using a one-chamber cell system at optimized concentration significantly lowered energy demand to 2.92 ± 0.04 kWhL⁻¹ of fuel produced, representing a 40% reduction compared to similar two-chamber setups.