Well, after recalibrating my speedo a long time ago, I later noticed that it's 4mph off.  So now, I'm wanting to recalabrate it better.  I'd rather have it read slow then fast.  So as I ask my friend who's a Service Manager at a Dealership in Socal,  if he can get me a 37T gear.   Well turns out there is no 37T.  It jumps from 36T to 39T.  Well taking into account of the actual tire size it could be a bit slower or faster.  And there is no 38T either.  What a PITA.  So I jsut ended up asking him to get me a 36T and a 39T.
I have a suspecision that my Jeep runs better when the speedometer is a bit slow then fast.