Hmm, some very interesting responses;
KciH, I measured the circumference of each, and they are almost identical.(the 18" I have has some wear on it)
Bonehead,...The 19" tire IS noticably lighter, just by picking it up!
I guess if I want to lower the final drive more, I will have to install a 54t rear sprocket, (longer chain required). I know some guys have used 12t counter shaft sprockets, but, I have been told that this puts too much stress on the counter shaft itself. Thanks Guys