Ratio Difference/Reduction
 

I have an 03 Burb 4WD with the 4L60E transmission, 3.73 gear.

Does anyone know, or is there some link/info somewhere, which will tell me the Gear Reduction or Ratio for the NP8 Transfer Case, when I put my Burb in Low Range ?

RPO NP8 - NP246 Transfer Case
 

Well, I guess I'll answer my own question...

The RPO for the Transfer Case is NP8.
That identifies the NP246 Transfer Case.
The Reduction Ratio for that Case, is 2.72:1.
Fluid changes for the Case are recommended for Normal Duty every 50K miles, or Heavy Use every 25K miles.
Proper Fluid for the Case is (only) Auto Track II, GM Part# 12378508.
A Vacuum/Suction Pump may be required to totally evacuate old Fluid.
The quantity of Fluid replaced should be just <2 Quarts, IF Case is properly drained.
Torque Drain/Fill Plugs to 15 lbs.
A few miles run in 4WD-Lo is recommended, then a re-check of Fluid level to assure proper content.
Replacement cost for this Case can run from $800 to $1200.

This is a link to a Parts Diagram of this Case (not to imply you should buy any parts from this Company)
http://www.drivetrain.com/parts_cata...rts/np246.html

FYI:
If you happen to have a 3.73:1 Axel Ratio and engage the Transfer Case,
this will equate to a ratio of 10.14:1 when in direct drive (3rd gear with the 4L60E transmission).

ratio
 

i was going to say most are around 2.72: 1 on almost any 4x4.

On my Rubicon its 4:1 for some serious rock crawling :)

Now; do u know why most ratios are odd numbers? axle ratios included?

there is a very specific reasoning...

DAHM !, Rocks and Trees as well :eek:. That'd put me at a 14.9:1 direct drive if I had that Case. Actually, I mostly do Mud, and Hi Range gets me through most of that. Though I like having that level of power, I really haven't ever needed it. The 5.3 is pretty torky. Is the Case you have in your Ruby original equipment, or a replacement ?

Theoretically, I think there could be even numbers, so, it must be one of those Engineering subtleties. That type Case is Chain Drive, and they probably could come up with any Ratio they wanted. But no, I really don't know.

Ratios
 

1. yes the Rubicon comes stock with a 4:1 case. A normal Wrangler is 2.72:1

the axles on the Rubicon are also 4.11:1. so, yea, it's LOW. Of course my 35" tires help too ;) (the 35s are not stock). (normal is around 3:1 or so, I never looked, I actually 'wheel, so Rubicon is the only way to go).


2. The ratios are odd numbered so that the teeth of the gears do not hit the same spot over and over with each rotation.

Think about a simple 2:1 ratio. If you marked a spot on each gear (where they mesh) those two spots would ALWAYS line up causing wear at those locations.

If you use an uneven number 2.33331587:1 ratio. The spots will only sometimes line up. Each tooth on the drive gear will touch each tooth on the driven gear. Thus spreading the wear across the entire gear.

isn't learning fun :)?

Also, when towing your buddy out of a ditch, or sand, etc. Use DRIVE on the towing vehicle. NOT reverse.

The gears in the pumpkins (diff) are cut in such a way that they are stronger moving forwards, than in reverse.

So, tow them out going forward if/when possible. Reverse risks breaking your ring/pinions

VERY interesting info thanks ;)
I've been pulling on wrenches a bunch of years, and there's always something one can learn :)

The "forward pull" issue is really good to know, hopefully this Thread will help someone out in more than one way. :D

PS:
The stock Tires on your Ruby were probably about 32" tall, so those 35's aren't that much larger. But, with the 4.11 gear, you probably have a fairly low OD ratio ? On the 4L60E it's 0.7, that really helps when you're just tooling around town.

Taller tires make the ratio smaller don't they?

Yes, they have that effect. I don't know very much about the Jeep line, but given the size of his Ruby, and with 4.11 gear, I bet his high (OD) gear is maybe as low as a .6. That'd put it a 2.46. Am I close SabrToothSqrl ?

Tires
 

Yes tire size comes into the equation; however it's more about ground clearance than the gearing. And off road, ground clearance is king. Why bash into it, if you can simply clear it?

Look up "portal axles" they are literally the most kick ass thing in the world.

(then look up Mercedes 6x6).


Generally, you have to gear lower (higher number) when you step up the tire size. However the the factory 4.11s in mine provide more than enough GO. Even with 35s. With the stock 32s, it must be damn scary fast. tire size doesn't change the "drive ratio", but in the real world, yes it does. You could divide the tire size by the size of the earth and that would give you the true 'final ratio'...

but you can also do simpler math using ratios that don't normally line up.

Like inches and ratios.


32" tires on 4.11 gears = 7.78 (let's call it "tire ratio")
35" tires on 4.11 gears = 8.51

so, to get back that missing 0.73 we need to gear down (bigger number).

35" tires on 4.56 gears = 7.76

So 4.56 gears get us back to within .02 of stock.

However, that's more cost /time / hassle than I'm planning on dealing with right now.

If I had 3.73 gears and went to 35s, it would be a different story. that would be a huge gap.

Gearing low (bigger number) = more torque, and (generally) less MPG.


Don't forget, all of these calculations are adjusted when you take the torque converter into account... that lets the engine spin faster than the transmission is actually going. Thus acting it's self similar to how a geared reduction works.

2012 Jeep Wrangler Rubicon Automatic Transmission Gearing:


2012 Transmission Ratios

Manual
1st 4.46
2nd 2.61
3rd 1.72
4th 1.25
5th 1.00
6th 0.797
R 4.06

Automatic
1st 3.59
2nd 2.19
3rd 1.41
4th 1.00
5th 0.83
R 3.16

so 3.59:1 then 4:1 then 4.11:1


puts me at: 59.02:1 in low range and first gear.
and 3.4113:1 in 5th gear.



Gear Ratio Calculator

(just multiply them all together since it's always a "to 1" ratio. and 1 * x = x.)

Although, the most 'off road' my Tahoe has been is the yard. I don't even know if I've ever had to use 4-low in it... the LTZ just isn't really an off road SUV...

That 59:1 is definitely impressive, I get about 31:1 with my Burb.
Thanks for the link !

I'm not trying to take anything away from dedicated owners, or improved Lubricants, but the inclusion of OD ratios, has been a major contribution to the longer (in miles) lives of our Vehicles. When you slow those RPM's down, they sure last longer.

I don't envy the GM Engineers tribulations, balancing that with the best gas mileage though.