I currently have 2008 Wrangler X auto (3.21 rear axle ratio). I have a 2012 Sport on order and I'm considering switching to a Rubicon. Does the 4.10 rear axle literally require the engine to rev 28% (4.10 divided by 3.21 equals 1.277) faster at highway speeds? [And, the 3.73 ratio would be 16% higher than the 3.21 using the same logic.] That would seem substantially "busier" than I'm used to. Also, any thoughts on the new 5-speed auto - is 5th going to be something of a super overdrive to help mileage?

 

As far as engine speed, you are completely correct: the 4.10 will rev ~ 28% faster and the 3.73 will rev ~16% faster with all other things constant (same tire size, same transmission, etc.) However I can tell you that the 4-speed auto currently in the JKs has a super high OD (like you mentioned in your post) making it so that 4th with the auto and 3.73 gears is equal in engine speed to 6th with the 6-speed with 3.21 gears. This is the reason why many hate the current 4-speed auto, because you are either lugging the crap out of the engine or you are in third gear revving the engine at well over 3K with no extra gear in between. This is also why many choose to re-gear so that 4th is more of that in-between gear and they may run their engine at ~2500 on the highway but that is better than always needing to downshift to over 3K with OD putting them at 2K...

So if 5th gear in the new auto is a similar ratio to 4th gear in the current auto (which I would assume it would be) then the engine speed will either be no different in OD and 3.73 gear ratios (compared to your RPM with 3.21 gears and 6th gear in your current JK) or only ~ 10% higher in OD if you get the 4.10 gears in the new 5-speed.

Hope this helps!