Gear Explained....

[steph74]

.... not by me, by you guys....

OK I am lost.... gears, ratio, high pinion, low pinion.... too many numbers. Can I have someone with a clear view putting all that together for me ? please ?

[LizardRunner]

I'll start with high pinion vs low pinion; A high pinion engages the ring gear above the axle center line, giving more ground clearance for the drive shaft. A low pinion engages the ring gear below the axle center line and puts the drive shaft at the bottom of the pumpkin, making it more vulnerable to rock strikes, it also increases the angle of the drive shaft which is not particularly good for long life of U-joints.

gear ratios are all about revolutions of the axles to the revolutions of the drive shaft. lower gears spin the tires more per drive shaft revolutions than taller gears. a 3.07 gear for example the drive shaft turns 3.07 times per revolution of the axle. The higher the ratio the taller the gears in hot rod speak. this is important to keep from straining engines when you increase tire diameter, taller gears keep the engine able to spin the tires with torque so you get your horse power to the ground, where you want and need it. so if you had 33" diameter tires with 3.07 gears it would take more of your engine to spin those tires than if you had 4.10 gears and if you had 4.53 gears it would take less effort from your engine than with the 4.10 gears. I think I have this explanation right but I know if I'm wrong I'll be corrected directly by those who know more about this than I do.

[steph74]

Thanks, I had that gear part about the same ;)

So for what I read there, what is the advantage of the low pinions ? It was my understanding that the newsest Cheokees came with low pinions ?
This taken from Wikipedia

Front Axle

1984–1996: Dana 30, High Pinion, Reverse Cut, 27-spline axleshafts (1989 – 1995 : with ABS used 5-297x universal joints, non-ABS had 5-260x universal joints. Certain XJ models were produced with constant-velocity joints instead of universal joints.)
1996–1999: Dana 30, High Pinion, Reverse Cut, 297x/760 universal joint, 27-spline axleshafts.
2000–2001: Dana 30, Low Pinion, Standard Cut, 297x/760 universal joint, 27-spline axleshafts.
1985–2001: Straight non-driven front axle for two-wheel drive only.

[steph74]

I have a manual shift year 89, Dana 35, i6 therefore, I can assume that my gear is 3.07

Now why would the manual ones have a much lower gear than the automatic ones for example ?
Still taken from Wikipedia
Axle Gear Ratios

Jeep XJs came in several standard gearing ratios:

3.07:1, manual transmission, I6 engine.
3.54:1, automatic transmission, I6 engine with Dana 44 rear differential.
3.54:1, manual transmission, I4 diesel engine with Dana 35 rear differential.
3.55:1, automatic transmission, I6, V6 engines; manual transmission, I4 engine.
3.73:1, automatic transmission, I6, Tow Package, UpCountry Package.
4.10:1, manual transmission, V6; automatic transmission, I4 engine.
4.56:1, automatic transmission, I4, offroad or tow package.

[XJ Rat]

If you want to know what ratio you have...

Jack up the rear of your Jeep. Mark the tire and mark your driveshaft. Spin the tire one revolution, while counting the number of times the driveshaft rotates. If the drive shaft goes just over 3 revolutions to the tires one, then you have 3.07:1 gears. If the shaft spins a little more than 3-1/2 times then you have 3.54:1 gears.

Or

Take off the rear cover and count the teeth on the pinion, then count the teeth on the ring gear. Divide one into the other, that will give you the ratio.

Or

Read the OEM tag under one of the bolts on the cover. It will tell you the ratio. That is, assuming it is the original tag and assuming those are the original gears. I took my tags off after installing a locker and new gear ratios in my axles. Some people won't.


The ratio in auto vs manual.
I will guess it has to do with losses in the transmission due to parasitic loss and slippage. The AW4 was less efficient than a manual tranny.


As far as the new Cherokees have low pinion Dana 30s. That was strictly a cost thing caused by bean counters (aka accountants). Dana ran through their stock of high pinion castings. Instead of buying more castings at a higher price, Jeep chose to install the same low pinion unit as the TJ's. There was a cost savings, buying in quantity. Jeep won, the consumer lost.

[LizardRunner]

I have a manual shift year 89, Dana 35, i6 therefore, I can assume that my gear is 3.07

Now why would the manual ones have a much lower gear than the automatic ones for example ?
Still taken from Wikipedia
Axle Gear Ratios

Jeep XJs came in several standard gearing ratios:

3.07:1, manual transmission, I6 engine.
3.54:1, automatic transmission, I6 engine with Dana 44 rear differential.
3.54:1, manual transmission, I4 diesel engine with Dana 35 rear differential.
3.55:1, automatic transmission, I6, V6 engines; manual transmission, I4 engine.
3.73:1, automatic transmission, I6, Tow Package, UpCountry Package.
4.10:1, manual transmission, V6; automatic transmission, I4 engine.
4.56:1, automatic transmission, I4, offroad or tow package.

The ratios of the transmission come into play with the differential gears. the manual tranny hits 1:1 at 4th gear. If you have a towing package,etc. they used higher diff. gears to make sure you don't blow up your engine when pulling something. And Jeep doesn't necessarily give you their expected setups. as an example I have a 95 sport with an optional upcountry (apparently, but it isn't on my build sheet as the upcountry package) because I sit about 1.5" higher than a same year stock but have no lift pucks, I have checked my gears and they are 3.73's, this has to be a weirdling of a jeep since I have the AX15 standard tranny.

[XJ Rat]

Yep, use your listing above as a starting point. I have a 1998. My window sticker and the factory build sheet clearly show the Jeep has the upcountry package, the tow package, and the AW4. My Jeep has factory 3.55:1/3.54:1 ratio axles. This does not agree with your Wikipidea list above. Best thing to do is check what you have.

[4.3LXJ]

I have a manual shift year 89, Dana 35, i6 therefore, I can assume that my gear is 3.07

Now why would the manual ones have a much lower gear than the automatic ones for example ?

Steph.

The difference is due to the different overdrive ratios between the transmissions. The idea is to get the same cruising rpm so that theoretically you get the same performance on the highway between the two. CAFE milage standards come into play here also along with performance.

[prerunner1982]

I'll start with high pinion vs low pinion; A high pinion engages the ring gear above the axle center line, giving more ground clearance for the drive shaft. A low pinion engages the ring gear below the axle center line and puts the drive shaft at the bottom of the pumpkin, making it more vulnerable to rock strikes, it also increases the angle of the drive shaft which is not particularly good for long life of U-joints.

The high pinion is stronger in a front axle application though a low pinion is stronger in a rear axle application. This is due to how the ring and pinion are designed. They have a drive side and a coast side.

In a front axle application a high pinion puts power to the drive side of the ring, but in a rear axle application it puts power to the coast side of the ring gear, which is weaker.

A low pinion in the rear puts power to the drive side, but in the front will put power to the coast side.

As you can see the drive side of the ring gear is flat (strong) where the coast side is angled.

http://www.differentials.com/images/toothterms.gif


I have a manual shift year 89, Dana 35, i6 therefore, I can assume that my gear is 3.07

Now why would the manual ones have a much lower gear than the automatic ones for example ?
Still taken from Wikipedia
Axle Gear Ratios

Jeep XJs came in several standard gearing ratios:

3.07:1, manual transmission, I6 engine.
3.54:1, automatic transmission, I6 engine with Dana 44 rear differential.
3.54:1, manual transmission, I4 diesel engine with Dana 35 rear differential.
3.55:1, automatic transmission, I6, V6 engines; manual transmission, I4 engine.
3.73:1, automatic transmission, I6, Tow Package, UpCountry Package.
4.10:1, manual transmission, V6; automatic transmission, I4 engine.
4.56:1, automatic transmission, I4, offroad or tow package.

The manual transmissions have a lower first gear and 5 speeds, where the autos have a higher first gear and 4 speeds.

1st gear:
AX15 AW4
3.83 2.80

2nd gear:
AX15 AW4
2.33 1.53

3rd gear:
AX15 AW4
1.44 1.00

4th gear:
AX15 AW4
1.00 0.753

5th gear:
AX15 AW4
0.79 N/A

The manuals have more speeds so that can have a lower gear to overcome the higher axle gears, where the autos have less speeds so they need a lower axle gear ratio to get the vehicle moving.

[Firemanray]

This link has some good information about Xj specs:

http://jeephorizons.com/tech/xjstockspecs.html

[steph74]

Good info there, thanks

[jimbosxj]

Hey, Being a tech and on my 5th xj/zj gearing and strength are relative with cost of what you want to do with any jeep and where you want to go with your xj. All frt.and rear ends work fine as long as they are not abused I've run 33 muds Km's on two of my xj's with a dana 35 with no issue's. My best advice is learn from other's mistake's don't think that there is one answer because there is not get as much info as you can and what you want to do and go for it. And always enjoy the ride Cheers

[steph74]

and this is the exact reason we stick on forums ;) learn from other people mistakes and have experienced people help us understand our mistakes ;)

[jeepxj95]

did the 8.25 come with 3.73 gears from the factory?? i have 3.55 in mine

[LizardRunner]

I'm pretty sure it came with them in it, the po doesn't seem to have done anything to it for wheeling, he drove it on freeways only. the build sheet says 3.07 but I counted the ring and pinion.

[steph74]

OK so Poking around, I found this thread/writeup on NAXJA.
http://www.naxja.org/forum/showthread.php?t=101355
I am assuming that if they put it up in the write up section of NAXJA, it has been deemed correct.
Basically that says that "Current tire size/new tire size =current gear ratio/new gear ratio"

SO.... let's see if I can get that correct in my application.

Current tire size: 235x75x15 or 28.9"
Current gear ratio: 3.07 (this being a manual transmission, I would have to check that it is stock)

If I want to put tires that are 33", according to that formula, I need (3.07*33)/28.9 = 3.50
So I am thinking that stock for automatic is 3.55 and that should be fairly easy to find then ;)

But then I had a doubt.... maybe I should use the stock tire size... so (3.07*33)/27 = 3.75....


So which reasoning is good if any ?

[4.3LXJ]

3.55 would be a cheap way to go. But don't discount the stock 4.10s you can also get. I run those on my 31s and love them. But they came with my Jeep.