To get a quick estimate of what a new sprocket will do for you, (or to you) you can use these calculations:

Speed at the same RPM:
speed/old number of teeth times the new number of teeth.
(100/16 * 18 = 112 KPH @ 4000)

RPM at the same speed:
RPM/new number of teeth times old number of teeth.
(4000/18 * 16 = 3555 @ 100 KPH)

This may not be exact, but it will be very close.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python