Fractional deviation from GR (Δσ_∥ = σ_{∥ ,MG}/σ_{∥ ,GR}−1) of the relative radial velocity dispersion, Δσ_∥, for the three f(R) models, binned by isolatedness (isolated vs. clustered) and mass (low and high halo mass) as described in Sect. 4.2. Pair separations r = 1 and 5 h^-1 Mpc are presented with bold and pale shading, respectively. The isolated high-mass halo bin was excluded for r = 1h^-1 Mpc because of the low number of halos within in.