Dirt Hammer

I'm sure most of you have beat this topic up pretty good but I would like to hear some fresh advise from everyone that's running 4.56 or 4.88 gears.

My Jeep is a 6speed manual 2015 JKU with 3.73 gears with 35in tires and aftermarket bumpers, winch, skid plates, 1ton steering, side steps, gear, etc. point is she is getting heavier as I build it out.

She is not my daily driver just a weekend Jeep that I use for exploring trails and backroads but I need to be able to run down the highway at 70 for hrs at a time to get to certain places, feared of over reving the engine at sustained speeds. Plus we are always up on the mountain roads of Georgia, TN, and surrounding states.

I'm not 100% trusting some of the charts I see floating around as some have not matched each other, I need to hear "seat of the pants" advise.

resharp001

So, I have a JKU 3.6L but an auto. Been on 3.73, 4.10, 5.13, and 4.88 spread across 35s and 37s. I live in Texas, but I head to Colorado to visit dad now and then. Mine is more of a toy as well, and I wheel it pretty good in the rocks. I will speak mainly in regard to RPM as that is constant across the tires/gears. For the last several years my tire size/gearing has put me at 3k RPM @ 70mph. I geared a bit low, leaning aggressive, to make up for the lack of a 4:1 rubi TC and to help with my wheelin’ style. The flip side of that was running higher than desired RPM’s at interstate speeds. I will say it was really nice to eliminate most downshifting on the highway, but I sacrificed fuel economy and driving 3100+ RPM on the interstate can get a little old with the 3.6L IMO. It also didn't help with my coolant temps, especially during the summers in Texas. It wasn’t the end of the world, but definitely something to consider based on use and geographic location. I just swapped axles and an now running more around 2750-2800 RPM @ 70mph. It sure feels better, but I’ve yet to be out wheelin it hard. I know I’m giving up a bit there.

Since you don’t have the 4:1 TC either, and since it’s just a toy, and since you are around more mountains, I’d lean just a touch lower than most maybe. I find the gear charts to be pretty accurate in general, but you have to use the ACTUAL tire measurement, not what is stamped on the sidewall. Guessing your 35s probably measure 34 if not 33. Based on manual transmission and tire size, my commentary would suggest 4.88’s.

I’m sure you’ll get some varying opinions.

Dirt Hammer

Thank you for the great deal of information. My tires are BFG MT and I believe they run in the 34.5in range. I really just looked at the 35 inch tire range on the charts, I'll go back and glance at the 34's and get a better understanding. 4.56 has been what I decided on but was thinking 4.88 would be sweet when in 4 high and 4 low.

resharp001

The obvious main difference in the equation is that you can keep it in whatever gear you want. It is so frustrating with an automatic, even if you throw it in "manual mode" that thing still downshifts on you and it can be so frustrating. I think ~2700 RPM is a nice sweet spot on the highway for the 3.6L.

Dirt Hammer

Yeah if I could keep it around 2700 at 70mph I'd be happy for sure. The current 3.73 just is not cutting it on road or offroad.

Dirt Hammer

Here's the chart I have been looking at. It indicates 4.88 with either a 34/35 inch tire the rpm should be pretty sweet at high 2600 or low 2600. What concerns me is I have had people tell me with 35 inch tires they went 4.88 and the rpm's ending up being 3k at 70mph. I only want to do this once so it needs to be right on.

resharp001

Keep in mind that the auto revs about 100 RPM higher than your manual so could be very likely seeing comments from people with autos as well. Also, people run various tire pressures and various load ratings which will impact actual tire measurements. Just saying, there are several X factors from jeep to jeep. I'd tell you that around Texas, 35s and 4.56 is standard sweet spot for the 3.6L. Going a bit lower to 4.88, especially on a non-rubi, is leaning a bit more aggressive, but is not terrible. It would be the equivalent of what I've dealt with the last several years. If you're doing hard offroading then the tradeoff is worthwhile. If you're wheelin fireroads and whatnot, maybe you don't need it. The X factor for you would be mountainous regions and how much time you spend in the hills. I would tell you that for around town and such, the lower gearing was nice. On the interstate when needing to pass, the lower gearing was nice. On the interstate and into a stiff headwind, the lower gearing was nice. On a 10-hour road trip, the lower gearing could be a drag.

nthinuf

If you are worried about the accuracy of the chart, test it. Measure your current tires and jump on the freeway, then compare. (For my 3.8 auto, those charts have been accurate through 3 ratios and 4 tire sizes. But, remember that it is only a number, many people forget to add in their specific variables for use and location and opinions on power they want, etc.)

As suggested above, use actual tire heights, not sidewall or spec'd height. 34.5" is the mfg spec, you are very likely to find that 33.5" is more accurate after you measure them..

Dirt Hammer

All good points guys and thank you. One other note on terrain, my Jeep is used mostly for exploring forest service roads, easy to mid trails but at times the terrain can be steep and ugly. I'm going to measure the tires this evening and start with that.

domano 68

Great information here! I am new so this helps a bunch.

Looking at a buying a used 2016 Willys with 6 speed manual and 4.88s. I will be using this for a daily driver for in city and highway driving. About 50/50. I do live on a mountain so I have to climb that coming home every time I would be driving it.

On the chart above, what does full time OD use mean please?

I do have some concerns here about top end on the highway and engine revs at 70mph. Any thoughts here would be much appreciated. Thanks!