I don't know about the skid plate, but I can answer the rest of your question. I've got a 2013 Rubicon 2-dr with the 6-speed (0.797 top gear ratio + 4.10 axle), and stock tires (255/75R17, so they're 32" in diameter, or about 100" in circumference). The automatic's top gear ratio is 0.83, so pretty close. I hit 2000 RPM at/around 58 mph. I've got an hour long freeway commute to work (not for much longer, hopefully!), and cruising at 75 mph, it's around ~2600-2700 rpm (it's a little hard to tell exactly, especially since I can't maintain speed for long with the traffic). The gear ratios came directly from Chrysler, here: http://www.media.chrysler.com/dcxms/...nltd_Specs.pdf (Transmissions for the 2013's are the same.)
The math seems to support our observations - at 70 mph, the tires should be turning at 739 RPM. (70 mi/hr * 5280 ft/mi * 12 in/ft / (60 min/hr) / (100 in/rotation) = 739 rotations/min). With your 0.83 top gear and 4.10 axle, your final drive is 3.40. That times 739 RPM is 2513 RPM, the engine speed you should be seeing at 70 mph. There's been a lot of rounding in this process, so I think your number is reasonable. My assumption would be that the short axle is aimed at customers who want to upgrade tires without regearing.
FYI, I'm seeing right at 18 mpg during my 45-mile, 75 mph highway commute (with about 4 miles of stop and go traffic at ends).
No your math is correct. I listed the wrong tire sizes in my first post. My 2013 has the stock 255 tires on it.
It just seemed like high rpm's to me. After looking at your post the rpm's seem to be normal.
Ah, just saw your correction - so yeah, I think it's where it should be. The 3.6 is designed to rev higher than the 4.0. (Redline on my 2013 JKR is around 6400, while my '98 Cherokee was around 5000, IIRC.)
The transmission gearing is different for 2012 and newer JKs, compared to pre-2012 JKs and TJs. The earlier transmissions had an overdrive ratio of something like 0.6:1. The new transmissions have an overdrive ratio of something like 0.8:1
My 2011 JK turned lower RPMs at highway speed with 5.38 gears in the axles than my 2012 JK does with 4.10 gears. The 4.10 gears border on being overkill with stock size tires. For someone that does a lot of highway driving 3.73 gears might be a better choice.
2014 Wrangler Sport: Teraflex/Synergy 3" suspension lift, Fox 2.0 reservoir shocks, Tom Wood 1350 driveshafts, Dynatrac ProRock 60s w/ 4.56 gears and Eaton electric lockers, Warn front locking hubs, Currie Anti-rock front swaybar, Fox ATS, 37" Toyo MTs on 17x8.5 Fuel Anza's, Poison Spyder bumpers and rocker guards, Warn M8000s winch, Teraflex tailgate hinge & spare tire carrier, Mopar head liner