The harmonic mean is the right average when you're averaging rates.
Formula
HM = n / (1/x₁ + 1/x₂ + … + 1/xₙ)
Try it
Drive 60 km at 30 km/h, 60 km at 60 km/h. Average speed?
HM = 2/(1/30 + 1/60) = 2/(3/60) = 40 km/h. (Not 45 — that's the arithmetic mean.)