Isobar spacing is really just a graphical way of showing rate of change of pressure over a given distance. More accurately, rate of change of distance for a given change in pressure.
The relationship between this rate and windspeed is dependent on the latitude at which it occurs.
Scales are available that are calibrated for the isobar spacing eg 2 HPa or 4 HPa etc, and the latitude.
Laying the scale across the isobars allows one to read the wind speed.
This windspeed presumes that no other effects are present eg surface friction, venturi effect through valleys, jets etc.
Some met. charts have the scale printed on them & you can use dividers or a ruler to measure the isobar spacing & then measure it against the wind speed scale.
[This message has been edited by Tinstaafl (edited 14 April 2001).]