I will always remember my last trip in my old heavily used 1991 Montero. That trip was to Scout Camp with KB on the East Fork of the Bear River in the Uinta mountains, the last week of July 2018. What a fun trip and great farewell to an old friend that served me well even though she was run down the day I got her over twelve years ago. The Montero ended up being the best $300 dollars I ever spent.
With the passing of my Montero I was in the hunt for a good 4x4 SUV that would take me into places like a side by side ATV. After a long and exhausting search I found that the best option for me was a used four door Jeep Wrangler Rubicon.
As I researched more and more into the Wranglers I knew I wanted one with a little lift, 33" tires or larger, a winch and the locker/sway bar options provided by a Rubicon. I also like the color white and preferred to have a little older vehicle that I wouldn't be afraid of scratching.
After a month or so I found the one I wanted in Logan and a trip or two to the credit union I had my first Jeep — a white with black hard-topped 2008 four-door Jeep Wrangler Rubicon.
I absolutely love the Jeep however after a couple of months of driving it I realized there was one thing that I hadn't researched about with running larger tires — axle gear ratios. The Jeep had 35" BFGoodrich KO2 35x12.5r18 tires which really only have a 34" diameter when new. My 4-speed auto-transmission with the 3.8 L engine and stock 4.10 axle gears just didn't have enough power to use fourth gear. Unless I was driving downhill I had to turn the overdrive (4th gear) off.
Weeks of studying gear ratio charts and talking with other Jeep owners I decided I better get the axle gears changed out. Wanting the jeep to last me for a very long time so I felt it best to get the gears changed over as soon as possible. I knew this had to be causing added stress to the transmission and engine by effectively only being able to use three gears of the transmission. I was running some higher than desired RPMs with only being able to use three gears instead of four.
The gear ratio I decided on was 5.13. This would take the gear ratio up three steps from the 4.10 stock gears that were in it. A lot of people tried to get me to go with the 4.88 gears but I really never felt this would offer enough power and I am so glad that I didn't get the 4.88 gears. One of the things I factored in going with the 5.13 over the 4.88 is that my Jeep has really heavy Rock Slider bumpers and a heavy winch. I also carry a lot of heavy tools and a high lift jack. With the heavier equipment, it further helped me decide to go with the lower gears.
I really like the 5.13 gears. I can travel up most interstates in forth gear even on climbs. Some of the steeper grades it does have to downshift to maintain 60+ mph.
If I could do it over again I would really like to try the next step lower 5.38 gears. 5.38 gears are the lowest gears you can fit in the Dana 44 axles that my JK Rubicon has front and back. One of the reasons I probably would prefer the 5.38 gears is when I get new tires. I'm not particularly fond of the KO2 tires I currently have. They are a good tire but I want a more aggressive mud-terrain tread on my next set of tires. I'm looking close at the Toyo Tire Open Country M/T Mud-Terrain Tires and from what I have been reading they are closer to 35" than my current tires thus lowering my RPMs a little from the 34" KO2 tires I am currently running.
Another thing to keep in mind is that I use a Superchips programmer to get the tires size and axle gears correct. I actually have the tire size set for 33.75" to get the correct MPH using my phone's GPS to verify my speed.
After 500 miles or so of driving it was recommended to me to drain the gear oil and replace it with some new oil. After a bunch of studying on gear oils I chose to use Lucas 85W-140 Heavy Duty Gear Oil. Researching gear oils I found that synthetic gear oil did not transfer heat away from the gears as well as traditional oil so I went with this Lucas 85W-140 Heavy Duty Gear Oil over the synthetic 75w-140 that the owners manual recommends.
RPMs going 70 mph in 4th gear with the stock 4.10 axle gears and 35x12.5r18 BFGoodrich KO2 tires.
RPMs going 70 mph in 4th gear with 5.13 axle gears and 35x12.5r18 BFGoodrich KO2 tires.
After 1,000 miles of driving with the 5.13 gears I drained the rear differential case. This was the horrible looking oil that came out. I got a little nervous that something was wrong. I then drained and replaced the front differential case and the oil looked the same color as well. A couple thousand more miles and I checked the rear oil and it is the pretty golden yellow color as it was when I replaced it. My guess is the shop used some funky conditioner or something that made the opaque silver color.