The Jeep Wrangler is one of two vehicles still being built with 
Mercedes' five-speed automatic (made by Chrysler in Indiana). The reason
 for that, according to Allpar sources, is the length of the ZF 
eight-speed which would be much better for Wrangler: it may fit into the
 Unlimited, but it won't fit into the standard Wrangler.
Even 
when the next-generation "JL" Wrangler arrives, that transmission will 
be too long, as it's made now. Yet, Automotive News and Allpar have both
 heard that the iconic Jeep will indeed have the eight-speed, which is 
practically mandatory if one wants to get the most out of the Pentastar 
V6 engine - especially if the Wrangler drops down to the 3.2 liter 
version, as we expect.
The solution may be to carefully alter the
 transmission and transfer case, according to former Jeep engineer, and 
current engineer-at-large, Robert Sheaves. See his illustrated article 
to learn more about how the eight-speed can be shoe-horned into even 
short-wheelbase Wranglers.
As read on: http://www.allpar.com/news/index.php/2014/12/fitting-8-speeds-into-wrangler