Electron acceleration efficiency for the radio relics of the sample vs. magnetic field in the downstream region. Calculations were performed using the Mach numbers listed in Table 3 measured in the X-rays (black) and derived from the integrated spectral index in the case of DSA (red). Lines denote the best fit Mach numbers (solid), the X-ray upper and lower bounds (dotted), and the upper (long dashed) and lower (dot-dashed) bounds of ℳ_DSA. For the Sausage relic, lines in blue represent assumed Mach numbers of ℳ = 2, 2.5, and 3 (from top to bottom).