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Ideal Cruising RPM for 4.0 from Gearing

39K views 31 replies 18 participants last post by  Jerry Bransford 
#1 ·
I'm looking into regearing my LJ. I am wondering what is the best cruising RPM that I should shoot for. Mainly mine is a DD and will see hunting trails. I do a fair amount of freeway driving as well. I'm running the 42 RLE auto (.69 final OD gear) on 305x55R20 (33x12's). OD is about useless as it can barely keep itself in on a flat. Current diff ratio is 3.73, and I"m aiming at a cruise MPH of 70. I'm thinking around 2200 to 2300 rpms for OD. Any input is appreciated!
 
#2 ·
Target 2700-2800. That's where I was sit on 35 (4.56) and it runs great. I've been on the full gamut of tire/gear combinations and this has proven the most ideal for me as a highway DD.
 
#4 ·
You have a manual, the OP has the 42RLE, different animal. It's that 0.69 OD that requires lower gearing (higher numerically).

OP, for 33's with the 6-cylinder and 4-speed auto the recommendation is 4.88.
 
#5 ·
go straight to 4.88s. do not pass go, do not collect $200. it will be better, but many say 5.13s would be better yet. 35s needs 5.38s or 5.89s. the deepest your D30 can go is 4.88.

the 42RLE's gearing is horrible, it comes undergeared from the factory. a 5-speed manual, with 3.73s and 35s turns the same highway RPMs as a stock 42RLE/3.73s and 30s.
 
#6 ·
Like everyone said, just go to 4.88 or get a waggy 44 for the front and drop in something deeper (Since you don't have the rubi lockers you can as high as 5.89, but 5.13 or 5.38 would be better for 33"s). Being undergeared from the factory sucks on anything but a flat or slight downhill highway.
 
#7 ·
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?


Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
 
#10 ·
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?

Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
Volumetric efficiency. An engine is nothing more than an air pump. Due to things like valve timing, valve configuration, port size and length, etc, etc. the engine will pump the charge in and out at the most efficient rate, at a given RPM. You usually see the VE

Couple that with the amount of power to keep the vehicle moving (gearing) and you produce a likable set up.

Not to be confused with peak HP and torque. If your building a dragster, you want the engine spinning at peak HP at the end of the run. You gear for that. On a torque vehicle, you gear to be driving at peak torque. Cruising you want the vehicle at the bottm end of the peak VE curve.
 
#8 ·
The RPM limit is well above 3K RPMS. Here's the peak hp / torque ratings for the six:

4.0L I-6 242 cu in - 2000-2006
* 190 hp @ 4,600 RPM
* 235 ft/lbs @ 3,200 RPM

The reason people shoot for 2600-2700 (or higher) is that's the spot in the power band that has the best efficiency, particularly with larger tires.
 
#9 ·
Rotational mass + brick shape + 4.0 = you need some RPM's to cruise efficiently and to provide the motor with adequate leverage. At 70 mph, the resistance against the shape of the jeep is working hard against you. IMO, the 4.0 needs to be at an absolute minimum of 2400 and preferably in the 2600-2800 range at 70 mph. Lower is not better in this case as you've seen with the 42rle lugger. The 4.0 reaches the point of peak volumetric efficiency at about 3100 RPM, so running closer to that is more efficient than lugging it. Regardless, you can't expect to get acceptable fuel mileage at 70 mph due to wind resistance, even if optimally geared. After about 60 mph, the dropoff is sharp.

As for factory rpm limit, not sure what you mean by that. If you're saying you never go past 3k, you may want to do that occasionally. Hell, I probably redline mine at least once a day....that's 5k.
 
#17 ·
Although my LJ is not a DD I find cruising w/ overdrive engaged at hiway speeds and flat areas not all that uncomfortable. I have 5.13s, auto, and 37s. My rpms at 65 - 70 are about 2100 - 2250. Frankly, I don't like running at 70 mph plus although the rig runs out smooth and occaisionally I do it...I simply didn't built it to be a hiway cruiser. Again, I'm not on the hiway much at all with the LJ and sometimes I just run w/ overdrive disengaged cruising closer to 2550 rpms which sounds/feels ideal and puts me at 55 mph, which is a safer "jeep" speed imho.

I'd agree w/ JB, ideal rpms for cruising is 2500-2700. Hopefully you geared for your desired road speed, per/ JB, and geared your transfer case to your wheeling needs.
 
#18 ·
For starters I did take the advice (thanks everyone) and went with 4.88's to match my 33s. I also went with true-tracs front and rear. (My wife can't drive a vehicle with lockers, and we have a lot of snow/ice in Nebraska) and I deploy frequently (got back from another tour 45 days ago). So she has to be able to drive it. I couldn't go any lower with the gearing due to the Dana 30.

I haven't done anything with TC yet... beings it's a DD I don't really wheel (no mud or rock crawling at all) in it, but I love it for a hunting/trail rig (plenty of deep snow.

I thought my Jeep was governing out at 3k rpm in OD (did that once) to figure out what it was.

I live where it's pretty flat with a few hills, mainly we get a really strong mid-west wind. I drive home (5 hrs) about once or maybe twice a quarter. The speed limit fluctuates between 60-75 mph. On average I stick between 70 and 75 on the interstate, I have no need to go any faster than that.
 
#24 · (Edited)
Glad you like the 4.88's because that is all that matters. Some folks might have similar or exact setups as you and their advice really helps while others have great experience and opinions. I've helped a few that PM'd me because I'm running exactly what they are thinking. Hell, I even sent them pics of my speedo and rpms to help them decide what is real life with their setup. :thumbsup:

Go drive that thing now and enjoy it!!
 
#21 ·
Ive got 4.88's on 33's and Im running 3K at 70 with the 5spd and get about 13 mpg, 4.10's maybe 4.56's would probably suit your DD needs better then 4.88's would and yes I know you have the AUTO so there is some differences but the 4.10's on 33's puts it back to a stock style set up at least for the 5 spd.
 
#22 ·
I really love how people post about what works for them without taking into consideration the needs of the person they are talking about. 5-speed and 42rle 4-speed auto.... very different things. I also love when people post about what someone else should do when that other person has already made their move.
 
#23 ·
LuckRider said:
I really love how people post about what works for them without taking into consideration the needs of the person they are talking about. 5-speed and 42rle 4-speed auto.... very different things. I also love when people post about what someone else should do when that other person has already made their move.
Reading comprehension?

Sent from my DROID2 using JeepForum
 
#28 · (Edited)
Any reason 3100-3400 at 75mph would be a concern?
Can this engine handle 3k+ for extended periods of time?
No and yes. Jeeps don't do well lugging the engine around at lower rpms. I have 5.13 gearing for my 5 speed with 35s. If you have a 42rle auto with the steep overdrive, 5.38 is a minimum for that tire size. It's not a sports car, it's a brick on wheels when driving at highway speeds. Ask Jerry B if you doubt it. His mpg's increased when he went from 4.88 to 5.38 with his auto on 35s.
READ THIS.
(75) Regearing from 4.88 to Revolution Gear's new 5.38. | Jeep Enthusiast Forums (jeepforum.com)
 
#27 ·
This is an old-fashioned engine that's not designed to remain lubricated at high RPMs for more than a few seconds at a time. If you drive on mountain freeways, you'll then notice the engine sounds different after a 30-second climb at 3,100 RPMs. By the time you reach the summit, the engine is losing lubrication from the extended interval at high RPMs.

Since I drive in fast areas, I kept it geared tall for freeway cruising at moderate RPMs. In city driving, the 42RLE automatically adjusts to the perceived lack of power by delaying upshifts. It will upshift at 2,500 RPMs now, whereas with factory tires it upshifted at 2,000 RPMs. So I'm keeping my engine in its power band longer for acceleration in the city. It feels better, and the guys who like high RPMs should be pleased. It should have come from the factory this way.

Internet forums have a perversity factor, where they advise you to regear, to run it at high RPMs on the freeway, so that you can run it at lower RPMs when accelerating and going uphill. It seems to me the conventional forum advice has it backwards. You should want lower RPMs for cruising, with higher RPMs being occasional instead of continuous.
 
#29 ·
Strictly from an RPM and engine/trans perspective. I went to 4.88 gears and I’m thinking 4.56 would have been a better choice due to the higher than expected RPMs. I swapped to the AW4 trans from the Cherokee. Overall I’m very happy with the results but I’ve yet to take it on a long highway drive and I’m worried the extra ~600 rpms at 75mph compared to the stock setup is going to blow it all up.
 
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