You didn't mention what your current gearing is, or what transmission you are running, but here is a very long message that I have been intending to type up for a long time. This is kind of a weird situation with our Jeeps, and after literally thousands of highway miles on my TJ back and forth across the country, I decided to stay with 4.10 gears in my TJR on 33 x 10.5s and, in most cases, I just turn the OD off.
Let me start with some background. My Jeep is at 145k, new water pump, new radiator, new hoses, new fan clutch, all within the last 20k. New cats within the last 10k, Banks cat back, new TPS, new idle air controller, new plugs (within the last 30k) No CELs - In my opinion, this engine is running tip top for what it is...
After dealing with an overheating problem on a trip out west, and generally just to pass the time on the highway, I obsess over watching engine load vs. engine temp. vs. intake air temp vs. RPM using the Torque app and an off brand bluetooth OBD reader.
With the 4.10 gears, anything over 65 or so, causes the OD to drop in and out quite a bit. If there is a head wind at all, or rolling hills, it gets pronounced.
When the OD engages, RPM drops to about 2000-2200 RPM. Engine load then begins to climb and the temps start to climb too. Not into a dangerous territory (usually), but they start climbing nonetheless. Engine temp climbs, and intake air temps actually start to go down. MPG climbs briefly, but begins to drop as engine load increases. I theorize that intake temps drop because the fan is spinning slower and moving less air across the radiator, and maybe the engine is just making less heat - The rotation of our fans literally throw air right at the air intake. (just a theory)
Once engine load reaches 100% the OD kicks out. Engine RPM goes up by about 1000 RPM. Engine load drops to about 70% depending on incline, wind speed, etc. MPG also goes up again briefly, but then comes back down and stabilizes. Coolant temp drops, intake air temps go back up (more engine speed = more coolant circulation, more air across the radiator, more heat thrown at the air intake)
The Jeep will run all day long, seemingly as happy as can be, up and down hills, a/c on, into the wind, whatever, at a very steady state, running in the area of 3000 RPM. So, in my head at least, that is the sweet spot.
So, then I started looking into regearing and using a calculator similar to the one referenced in this thread.
At 75mph, with 33" tires:
4.10 gears = 2160 RPM in OD, 3131 RPM out of OD
4.56 gears = 2403 RPM in OD, 3482 RPM out of OD
4.88 gears = 2571 RPM in OD, 3727 RPM out of OD
And that is as far as I looked. But notice, in every scenario, I never get back to that sweet spot of 3000 RPM in OD, but the engine will really start to scream when the OD drops out. 4.88 gears will get me close, and might do the job, but should I come to a hill or need to squeeze the throttle a bit more, the OD will kick out, the engine will howl. RPMs will actually jump to the backside of the torque curve. I have a friend who usually travels with me in a 4 door JKR who has regeared and is dealing with this exact problem - not enough engine speed in OD, but too much when the OD kicks out.
So in the end, I am staying with 4.10 gears and I just habitually turn the OD off when I start up the Jeep when I am running long stretches of highway. If I notice a tailwind, or am running a downhill section of highway (that isn't too steep) I click the OD on and take advantage of whatever MPG I can get. I'll usually leave it in OD until it drops out on it's own and then I manually disable it again. Having it kick in and out seems like it would cause the most wear and tear and is frustrating when trying to use the cruise control, as the speed just begins to fluctuate too much as the OD drops in and out.
I'm not sure if that is the perfect answer, but I'm just not willing to gamble the expense of regearing to find out, because this is one of the more expensive mods we take on in our Jeeps.
The real magic bullet for the TJs with auto transmissions would be a less deep OD gear. The .69 OD really just cuts way too much RPM for our engines.
Long winded, and it kind of goes against the general line of thinking in the Jeep community, but it works for me. I hope it gets you somewhere.
-T.
