Wow, thank for all the info covering a lot of areas that I will have to address to properly build my Jeep.
I'm from Sri Lanka and access to heavy duty Jeep parts is very difficult and I will have to import components myself. With a very weak currency and heavy import duties, it is also double the cost of the parts themselves usually. We are also Right Hand Drive making some front axle related components like trac bars and steering components hard to find.
I'm planning to hold onto this Jeep and build it gradually. I really want to sit on 35s. First I'm planning to upgrade the tie rod and drag link along with HD ball joints which. Then to get Metalcloak arched fenders and get the 35s under the jeep and regear at that point. I will then look at replacing the 1 inch suspension and 2 inch body lift that the PO put on with hopefully a 4 inch Savvy short arm kit and Fox shocks. Axle shafts will be next. I'm gentle with the Jeep for the most part so I hope the stock shafts will hold for a while.
back to gears, wouldn't 5.13s wind up the revs too high for daily driving? I understand that it will be desirable in a dig offroad. But would you still say it is the right ratio for a Jeep that spends 90% or more time on the road?
Many thanks for your input
5.13 gears are intended for the road, and also happen to be good off-road. The 42RLE has a much taller OD than all other TJ transmissions, meaning at highway speeds the engine turns much more slowly than it would in a 5/6 speed manual or 3-speed auto TJ.
That tall OD works wonders for fuel economy in most vehicles, as the power demand at highway speeds is generally pretty low.
However, the TJ has the aerodynamics of a brick, and with large tires has a high rolling resistance. Thus the power demand at freeway speeds is much higher, and the engine needs to provide more power.
Engine instantaneous power is proportional to the instantaneous torque times the revolutions per minute. So to increase power you either increase torque (by opening the throttle more) or RPM (downshifting or unlocking the torque converter).
As such, the TJ ends up unlocking the torque converter or frequently downshifting to maintain speed, far more so than most cars.
By regearing to 5.13, you are getting closer to the optimal RPMs in OD necessary to sustain highway speeds. As such, the car will downshift less often, and unlock the torque converter much less often. When the torque converter is unlocked, a significant percentage of the engine’s power is lost as heat in the transmission, reducing fuel economy. When the torque converter is locked, it has roughly the same efficiency as a manual transmission. So generally by gearing in a way to reduce the amount of time the torque converter spends unlocked, you increase transmission efficiency.
As for engine efficiency, this vastly depends on the load demanded by the engine. An engine is most thermodynamically efficient when at WOT at the peak torque RPM (about 3500 RPM). Below that power demand, the peak efficiency at reduced power scales roughly linearly (a bit more complex, but I won’t go into it too deep) with RPM and ideal demanded torque (throttle position). Since the power demand (at a given highway speed) as a proportion of engine capacity of a TJ is higher than say a Ford Mustang, the ideal RPM for cruising at highway speeds is significantly higher.
Finally, the deeper gearing will help protect the engine and transmission and potentially give them longer service lives. When the torque converter is unlocked, all that heat has to go somewhere. Generally what happens is the transmission significantly heats up, resulting in lower life, and higher odds of failure. In the engine, the lower combustion temperatures lower the temperature of the cylinder walls, helping to reduce engine temperatures. Additionally, in both the engine and the transmission, the water pump/cooler pump spins faster, resulting in a more stable engine temperature and cooler transmission temperature.
As RPMs increase, there is some additional energy loss. Not only do dry friction losses scale linearly with rotational speed, viscous frictional losses scale exponentially with speed. At low to mid RPMs, these losses are counteracted by the things I mentioned above. But at higher RPMs, the exponential losses tend to dominate. Once you pass the peak torque RPM (3500 for a TJ), you will not see any additional efficiency benefits by increasing RPM, and you will see losses instead. At lower power demands that threshold is lower, but it scales with power demand as a proportion of engine capacity.
That said, you still get more power at higher RPMs than peak torque, up to the peak horsepower RPM (4600). It just comes at an ever increasing cost as RPMs rise. So climbing a hill or passing someone, it is generally fine to run the engine up to redline before passing , but for economic reasons (as well as wear and tear), it is not reasonable to indefinitely run at or near redline.