With the geostrophic wind associated with straight, parallel isobars the pressure gradient force (PGF) is balanced by coriolis force.  

Coriolis =2ωρVSinLat where

ω =rate of rotation of the Earth, ρ =air density and V = wind speed

So PGF = 2ωρVSinLat therefore V .    PGF      .

                                                               2ωρSinLat

 Wind speed varies directly with pressure gradient force and inversely with the rate of rotation of the Earth, air density and the sine of latitude.