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Old 10-14-2010, 06:44 PM   #1
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Couple of TJ questions

Hey everyone. Couple questions. I've been reading around here and people are saying that they run their Jeeps at around 2700 rpm all the time? They say that it's "best" for the 4.0. I want to know how much truth there is to that. I generally shift around 2000 rpm or so and the engine doesn't normally bog down unless I shift slowly or am going up a hill so I run about 1600-1800 rpm normally and 2100 on the highway at 60mph. I just don't see how running at 2700 can be good. I decided to break 2000 the other day and it sounded like the Jeep was absolutely screaming. Maybe its normal and I'm just not used to it but I cannot comfortably run my Jeep at 2700 and I don't know how some people here do it.

Next question. How can I easily tell what my axle gearing ratio is with out jacking up the back end and spinning a tire? I looked at the dif cover today and saw a few numbers but nothing that represented a gear ratio.

Final question. What are all the parts needed to do a 2" BB. I know it has the spacers but beyond that I have no idea. Also how much will a decent BB cost me?

tl;dr: mystified by cruising rpms, gear ratios, and BB questions.

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Old 10-14-2010, 06:56 PM   #2
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I have an automatic so I can't really help with the RPM question.

Your gear ratio is on the metal tags on the axle housing, post up what numbers are there and we can tell you.

For a 2" BB you will have the spacers, shocks, extended bump stops may come with it too. I would get some quick disconnects to go along with it. If you buy it as a complete kit you will be good to go.

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Old 10-14-2010, 06:57 PM   #3
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I have owned 4 TJ's never heard of the 2700rpm thing. As for gears, write the tag numbers down and post.. what trans do you have and what engine? for 2"bb, spacers, shocks and t-case drop maybe(each jeep is different if it needs one or not) figure $200-$300 depending on shocks and which brand you buy.
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Old 10-14-2010, 07:02 PM   #4
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People coming from cars are used to low engine rpms and those rpms are too low for a Wrangler due to its heavier than average weight, aerodynamics, and generally large to extra large tires.

And if you're referring to my earlier comments when you ask how much "truth" there is to them, I can only say I said at least 2400-2500 rpms are appropriate on the highway for most Jeep size tires but higher rpms like 2700 are more appropriate for larger tires like 35". The ratio most often recommended for 35" tires and the 5-speed by experienced builders and offroaders is 4.88 which produces about 2700 rpms on the highway. That is the same rpm produced by the commonly recommended 4.56 and 33" tire combination which is equally recommended when the transmission is the 5-speed manual. I'd only try to get away with rpms lower than 2400 if I was running the smallest factory size tires.

2700 is nothing for the 4.0L engine and it'll run that all day every day without even realize it's working hard. I ran 3000 rpms on the highway after converting my 5-speed manual to the 3-speed automatic with my 4.88 axle ratio and 35" tires and it was fine too... once I got used to it.

Don't get confused by what rpms your car engines run at while running tiny tires, a lighter weight vehicle, and sleek aerodynamics. Apples and oranges.
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Old 10-14-2010, 07:03 PM   #5
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its the last three digits on your diff tag

and t-case drop is cheap if you need heck i got one i can't give away

and you should do longer shocks if you get any kind of suspension lift period.
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Old 10-14-2010, 07:07 PM   #6
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Quote:
Originally Posted by EdJonesJeeper View Post
its the last three digits on your diff tag.
The ratio is actually a separate group of three numbers in the middle of the tag. It's not the last three digits.

The below tag shows its 3.73 ratio inside the red circle.
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Old 10-14-2010, 07:20 PM   #7
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I have to say that the 2400-2700 range sounds very high, I run 235/75/15 (29's) with 3.73's and tend to stay right around 1800-2000 in town and in fifth at 55 I run aprox. 2100-2200. And I can also say that when i run it up into the 2500-3000 range it does not sound like that is where it wants to be. It does scream up in the 2600-3000 range, normal gear whine and just RPM'S but it does not sound like a cruising RPM at all. I have been driving sticks for years and it is the sound that says " time to shift"

Is all this advice directed at people with at least 33's or am i missing something?
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Old 10-14-2010, 07:27 PM   #8
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Thanks for all the replies everyone. The rpm issue was being discussed HERE and I believe that's where Jerry posted. Man I guess moving from an automatic Ford Expedition which rarely went over 2500 rpm when passing on the high way to a manual Jeep Wrangler is non comparable and two completely different caliber vehicles. I still cant wrap my head around how it can run at 2700 rpm. That means if I wanted to I could cruise 25mph around town in 2nd gear at about 2300 and still be fine. Of course I have fairly stock tires on there right now. I think they're 265's? I can't remember off the top of my head but I want something bigger. It's just weird lol.

As for the diff cover I didn't notice a metal band on it like in the picture above. Then again I may have just not known what I was looking for. Tomorrow when it's light out I'll check it out. The only obvious numbering I saw was actually casted with the cover. Like I said I'll check it out more closely tomorrow.

I have an 04' Wrangler Sport with the 4.0L and the NV3550 5 speed with the Dana 44 rear axle (I have no idea if its limited slip or open). Should I be alright with 31x10.5xR15 Duratrac's and a 2" Budget Boost without a T-Case Drop?

Thanks for all your help guys.
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Old 10-14-2010, 08:44 PM   #9
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Quote:
Originally Posted by Jerry Bransford View Post
The ratio is actually a separate group of three numbers in the middle of the tag. It's not the last three digits.

The below tag shows its 3.73 ratio inside the red circle.
Nice been a while since i looked, I lost my tags long time ago
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Old 10-14-2010, 08:45 PM   #10
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If you can't see a tag on your rearend then you can go to the local dealership for jeeps and give them your vin number. They will print out a build sheet for you with all the information. Or you can to go Official Jeep Site - 4x4 SUV, Sport Utility Vehicle and go to contact customer assitance and click on email us.... put in your information and ask for a build sheet.

I also run close to stock tires. 235/75R15s here. I usually shift around 2100 RPMs also and normal driving in town is around 1800 to 1900. On the Highway its from 2100 to 2300, depends on what the speedlimit is on that highway. My gearing is also at 3.73 with a 4.0.
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Old 10-14-2010, 10:51 PM   #11
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Thanks for chiming in scotiez, nice to see that your number's are right in line with mine..I was a bit confused after the talk of much higher cruzing rpms
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Old 10-14-2010, 11:29 PM   #12
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Quote:
Originally Posted by InvertChaos View Post
I have an 04' Wrangler Sport with the 4.0L and the NV3550 5 speed with the Dana 44 rear axle (I have no idea if its limited slip or open). Should I be alright with 31x10.5xR15 Duratrac's and a 2" Budget Boost without a T-Case Drop?

Thanks for all your help guys.
I'm gonna guess that with a D44 you have 3:73 gears, but that is just some random crap I pulled outta my pickled brain.. You should be fine with 2" bb and no tc drop. Make sure you do an alignment after the lift. Even 2" throws stuff out of whack..
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Old 10-15-2010, 08:01 AM   #13
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I have a Mazda with a stick. The most comfortable RPM range for it cruising on the HWY is 3,000 RPM. In fact a little less then that and it almost feels like it's going to bog down if I need more speed. My TJ is a 4.0L and it really likes to cruise at 2,500 RPM....
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Old 10-15-2010, 08:23 AM   #14
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I'm sure we've all punched it on the highway at some point and I've deffinatly noticed that the jeep pulls a lot better between 3 and 4000 rpm the closer you are to the peak power and torque, the less your engine has to work to maintain whatever speed your traveling and when you have to push the go pedal a little more it will actually respond. going up a slight grade @ 65 mph (20-2100 rpm 5th gear 3.07/8 30" tires) in my jeep I put my foot to the floor and it slows down.
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Old 10-15-2010, 08:33 AM   #15
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Originally Posted by InfernoGirl
I'm gonna guess that with a D44 you have 3:73 gears, but that is just some random crap I pulled outta my pickled brain.. You should be fine with 2" bb and no tc drop. Make sure you do an alignment after the lift. Even 2" throws stuff out of whack..
I have a 97 sport, 5 speed, D44 with limited slip, and for the longest time thought I had 3.73s. I checked my tag last week while considering a possible regear after 8 years of running 33s, and surprise! I have 3.55

This rpm discussion has definitely given me something to ponder, and maybe my lack of power is more my driving style based on when it head stock - vs lifted, weighted by bumpers, big spare, and other gear, and 33s
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Old 10-15-2010, 08:43 AM   #16
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I have a 97 sport, 5 speed, D44 with limited slip, and for the longest time thought I had 3.73s. I checked my tag last week while considering a possible regear after 8 years of running 33s, and surprise! I have 3.55
Yeah, I forgot that in '97 they used 3:55's in some of them... Confused as to why having 3:55's makes you when you thought you had 3:73's tho? Lower gears (higher #) are better for bigger tires.
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Old 10-15-2010, 09:05 AM   #17
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I shift gears at 3000 RPM all the time. That is all I have to add to this.
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Old 10-15-2010, 09:16 AM   #18
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The motor is underpowered. Yes if you wanted to keep the same motor, the way to get it to run lower RPM happily would be to put it in a lighter, sleeker vehicle. But the 5.3 in my old 5000 lb, brick wall of a pickup would run at 2000 RPM at 70 mph all day up hill (running 30" tires). A powerplant more suited to this vehicle would be able to push it at lower RPMs despite its weight, aerodynamics, and tire size.

That said, I'm running in a similar RPM range as has been mentioned (mid-2000s) with a stock '06 LJ (4.0, 30" tires, 6 speed, 3.73s)...they're built for it...
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Old 10-15-2010, 09:50 AM   #19
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I drive over 3000 rpms on the freeway every day (6 speed, 4.10s and 31s). 6+ hr freeway trips at 3100rpms. I never ever shift below about 3000 unless i'm going down hill. I've tried the "shifting under 2K" and it doesn't work for me. Cars go flying past me like i'm stopped. I shift above 4000 any time i'm trying to accelerate at a decent rate.
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Old 10-15-2010, 10:00 AM   #20
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Quote:
Originally Posted by InfernoGirl

Yeah, I forgot that in '97 they used 3:55's in some of them... Confused as to why having 3:55's makes you when you thought you had 3:73's tho? Lower gears (higher #) are better for bigger tires.
Not smiling at the gearing I was dealt...smiling at the fact that i was blissfully unaware of exactly how bad it was!
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Old 10-15-2010, 10:23 AM   #21
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Oh, I see!
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Old 10-15-2010, 11:01 AM   #22
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Old 10-15-2010, 11:04 AM   #23
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One more note, the correct rpm also depends on your transmission. With the newer 42RLE 4-speed automatic transmission, it needs a much lower axle ratio to run properly than, for example, you do with the 5-speed manual transmission.

When I had my '97 TJ with its 5-speed, the 4.88 gearing I ran with 35" tires was ideal. Highway rpms were where they needed to be for such a big tire, and they were just right for offroad too. But since that Jeep was stolen and I bought the '04 Rubicon with the 42RLE automatic, its 4.88 gearing is not low enough to run my 35" tires. The engine lugs and the transmission is constantly hunting for gears... it's hard to get into Overdrive too. I am constantly having to turn the Overdrive off, especially for even a minor grade. For that particular transmission, 5.13 is the minimum gear with 5.38 being the one commonly recommended. The guy I bought it from, Gerald who co-owns Savvy Offroad, told me he regretted having regeared it to only 4.88 and I agree. 4.88 with 35" tires and that particular transmission is barely ok on the highway, 5.13 or 5.38 would be much better.

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