Disconnect the VSS connector and check continuity of the black wire it must show continuity with both the engine and body ground under all conditions.
Next check voltages on the other two wires in the VSS connector with body ground, one will show battery volatge (12 V) and the other will show around 5V.
Perform these tests under all three conditions i.e.,
a. ignition switch ON (engine off)
b. engine running in petrol
c. engine running in CNG