Using synthetic absorption lines generated from 3D hydro-dynamical simulations we explore how the velocity of a starburst-driven galactic wind correlates with the star formation rate (SFR) and SFR density. We find strong correlations until the scaling relations flatten abruptly at a point set by the mass loading of the starburst. Below this point the scaling relation depends on the temperature regime being probed by the absorption line, not on the mass loading. The exact scaling relation depends on whether the maximum or mean velocity of the absorption line is used. We find that the outflow velocity of neutral gas is four to five times lower than the average velocity of the hottest gas, with the difference in velocity between the neutral and ionized gas increasing with gas ionization. Thus, absorption lines of neutral or low ionized gas will underestimate the outflow velocity of hot gas, severely underestimating outflow energetics.
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