Worked Example
From the diagram below the large cog which is the one being driven has 40 teeth.
The small cog (which is the one that is doing the driving) has 8 teeth. If we substitute these into our formula :
          Driven
VR= -------  therefore :
            Driver
          40             5
VR= ---  =  --  therefore 
        8          1
the VR= 5:1 notice that the figure above the line comes first. (fig.1)

If we make the small cog the one that is being driven the calculation looks like this:

        Driven           8          1
VR= ------    =  --    = ---  therefore the
        Driver            40        5

VR = 1:5  (we still put the figure above the line first) (Fig 2)

This means that for every 5 times the driver goes round the driven only goes round once. In this case the motion is being decreased by a factor of  5.

Driver goes round once, driven goes round 5 times, increasing the speed by a factor of 5.

Important If the two wheels were pulley we would use the sizes of the wheels in the calculation instead of the number of teeth.