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High Elevation Gear Ratio - 33's - 6-Speed (NSG370)

PowderFactory

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Okay, I know there are a million posts about gearing (and I think I've read them all) but here is another! I could really use some last minute advice before I get my Jeep regeared. I am currently running 33" takeoffs, and may go up to 34" tires (285/75/17) in the future when the current tires wear out. No plans of going any larger than that, as I want to stick with my current lift (2"OME, 1" BM and MM). I have the 4.0L, NSG370 6-Speed, and 3.73 gears in the Dana 30 and 44, and currently my LJ is way undergeared! I have had 4.56 and 4.88 gears recommended by others. The online charts / grimjeeper / forum members usually recommend 4.56, but I live in Colorado at over 9000', and spend most of my driving is in the mountains at high elevation. Most 4x4 shops in Colorado have recommended 4.88 (due to the altitude and hills). The Jeep is my DD, and I do 80% highway, 20% trail (no extreme rock crawling). I want to be able to go 65mph up mountain hills on the highway, and still be able to go 80mph down hills without revving crazy high. Medium weight Jeep (hard top in winter, might add steel winch bumper, etc). Any suggestions or personal experiences with similar set-ups would be greatly appreciated!

jeep.jpg
 

JMT

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I’m happy with 456 and 33’s with the 6-speed in my TJ. If you are going to a bigger tire the decision is easy; 488.
 
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Artsifrtsi

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I don't know why my NSG370 with 3.07's and 31's feel appropriately geared with all the talk of steeper gears... I can still accelerate nicely, I pull all the hills in my area in 6th gear at 65-70, and do well offroad. When I go up to 33's I'll probably run the gears up to 4.10's...
 

NOTNSUV

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I don't know why my NSG370 with 3.07's and 31's feel appropriately geared with all the talk of steeper gears... I can still accelerate nicely, I pull all the hills in my area in 6th gear at 65-70, and do well offroad. When I go up to 33's I'll probably run the gears up to 4.10's...
Driveways in the west are steeper than "hills" in 'Bama'.😃
 

Rickyd

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Lower gears won't help a lot in the power dept. They will make overdrive usable though. Higher compression ratio,stroker or forced induction would give you more power at altitude
 
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PowderFactory

PowderFactory

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What is your idea of "revving crazy high"?
Good Question! I’ve heard the 4.0 does well at high rpm, but the crawl calculator shows 3,340rpm in 6th at 80mph w/ 4.88’s, and 3,120rpm w/ 4.56’s. (Currently I only hit 80 going downhill with the wind at my back, which is part of the reason I want to regear) Going 65 up steep mtn hills on the highway is the main reason I’m upgrading (Currently I can only go 50mph up some hills with the stock 3.73’s, bouncing between 3rd and 4th gear)
 
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BlueC

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I’ve heard the 4.0 does well at high rpm

I'd be inclined to say no, no it does not. The 4.0 I6 in our jeeps is a lumbering engine, making its power in the low-to-mid RPM range.

People like to call engines like that "torquey" to make themselves feel better. :LOL:
 
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rasband

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I'd be inclined to say no, no it does not. The 4.0 I6 in our jeeps is a lumbering engine, making its power in the low-to-mid RPM range.

People like to call engines like that "torquey" to make themselves feel better. :LOL:
2800-3200 is peak I believe.
 
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JKP

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Good Question! I’ve heard the 4.0 does well at high rpm, but the crawl calculator shows 3,340rpm in 6th at 80mph w/ 4.88’s, and 3,120rpm w/ 4.56’s. (Currently I only hit 80 going downhill with the wind at my back, which is part of the reason I want to regear) Going 65 up steep mtn hills on the highway is the main reason I’m upgrading (Currently I can only go 50mph up some hills with the stock 3.73’s, bouncing between 3rd and 4th gear)
For '06 the 4.0 specs are:

190 HP at 4600
235 ft lbs at 3200

Note that 3200 is straight up on the tach. My 92 Jeep tach is straight up at it's torque peak as well.

It will be fine running 3100 to 3300 all day long and you will be at or near the tq peak.
 

JMT

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What does altitude have to do with it? These Jeeps are injected. I run all over southern Colorado mountian passes and am running 456's, 33's and the 6 speed. Never had a power issue, only problem is the Jeep tours going the wrong direction on the narrow pass roads.
Only difference is this guy has an LJ. I don’t know how significant the weight addition is, but I agree I have no problems on hills in the Rocky Mountains with 456’s and 33’s with the 6-speed. If he’s planning 34’s I’d go with 488, which is what I should have done, though I am still happy with 456’s and 35’s now.
 
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Steel City 06

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While normally I would advocate for a deeper ratio, consider that the NSG370 is a bit different. With 4.88, you have two usable freeway gears, 5th and 6th. But on 4.56, 4th also becomes significantly more usable.

Think about it this way. At 60-80 mph:
6th: cruising gear
5th: torque gear
4th: power gear

With 4.56 and 33s, 4th gear is in the powerband at freeway speeds (around 4500 RPM). Since it is so close to peak power, 4th gear will deliver the most torque to the ground in that combination, allowing you to keep up and even pass people at 75 mph.

Go to 4.88, and you’re beginning to overrun 4th at freeway speeds, and you’ll be more reliant on 5th gear on freeway hills. At 4.88, you wouldn’t have nearly as much power in 5th as you would with 4.56 in 4th. You’d have to go on the order of 5.38 to make 5th almost as useful as 4th with 4.56.
 
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PowderFactory

PowderFactory

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While normally I would advocate for a deeper ratio, consider that the NSG370 is a bit different. With 4.88, you have two usable freeway gears, 5th and 6th. But on 4.56, 4th also becomes significantly more usable.

Think about it this way. At 60-80 mph:
6th: cruising gear
5th: torque gear
4th: power gear

With 4.56 and 33s, 4th gear is in the powerband at freeway speeds (around 4500 RPM). Since it is so close to peak power, 4th gear will deliver the most torque to the ground in that combination, allowing you to keep up and even pass people at 75 mph.

Go to 4.88, and you’re beginning to overrun 4th at freeway speeds, and you’ll be more reliant on 5th gear on freeway hills. At 4.88, you wouldn’t have nearly as much power in 5th as you would with 4.56 in 4th. You’d have to go on the order of 5.38 to make 5th almost as useful as 4th with 4.56.
Thanks Steel City 06, I appreciate your well thought out answer! Most of the advise I have received is from 5-speed TJ and Auto owners, and the 6-Speed NSG370 seems to have pretty different gear ratios!
 

Steel City 06

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Thanks Steel City 06, I appreciate your well thought out answer! Most of the advise I have received is from 5-speed TJ and Auto owners, and the 6-Speed NSG370 seems to have pretty different gear ratios!
Note the above logic applies only if you’re not afraid to rev your engine. I run it up near redline on a regular basis when I need the power, such as to hold speed on a steep freeway hill or pass an elderly driver. In a select few cases, I may even do something as big as a 6-3 downshift.

Don’t be afraid to downshift as far as is necessary to climb a hill or accelerate. As long as you aren’t holding near redline RPMs for more than a few minutes at a time or exceeding redline, an engine in good condition should have zero issues. (In fact, it actually helps knock off and burn up carbon deposits when done occasionally.)
 

Mudflat

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I’m happy with 456 and 33’s with the 6-speed in my TJ. If you are going to a bigger tire the decision is easy; 488.
I have the same engine & transmission as you & I also like 4.56 gears. I live on pretty flat ground nearly at sea level (southeast Texas). For what I do, I don’t think I would go any deeper than what I have.
 
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