Swapping from 3.73 to 3.42 Gears

[Lightman]

That makes more sence

 

[AKaiser]

I think some one is feeding you the wrong numbers to get 120 teeth on the ring gear you would need a housing 3 times as big as we have. pull the cover and count them there will be a difference from 4. 10 to 3. 73 but not any where close to 120 teeth more like in the 30-40 something range just a guess on the number

IIRC it's like 32 or 33.

 

[JACummins2500]

I'm going to guess they are, based on a discussion I had online with a Dodge rep. She did some research and concluded that you could now get 4. 10s with a G56. Apparently the website vehicle configurator is incorrect. We shall see.



Based on the Klenger gear caluculator, I figure, with 3. 42 gears, I could tow anything I needed at 55 in 5th with the stock 265-70R17s. Any daily driving I could run the 33s I'm using now, and gain a 300 RPM decrease (over stock tires/3. 73s) at interstate speeds (70 in IN). It would be nice to have taller 5th and 6th gears and 3. 73s, though.

I agree with you that all that you need to do is drop back to 5th. However, Dodge/Cummins has to figure that some people would not if they sold it that way. Once again, we all have to suffer due to the lowest common denominator.



Cowboy Cummins. I don't like to drive at 55 either. However, sometimes when traveling on a two lane hightway, it cannot be avoided. 83 mph only last for about a mile and then you always encounter someone I guess Dodge/Cummins figures I am not the only one.

 

[Cummins Cowboy]

Is that verified by your overhead display, needle position, or seat of the pants?

the overhead display is a joke, I am usually about 3mpg below it, its so bad I don't know why they put it on the truck, I filled up this morning and hand calculating proved my thinking I am up at least 1. 5 mpg, which when pulling a trailer for work almost everyday and being in the low tens is nothing to sneeze at, it amounts to about a 10% increase. I think I would see larger increases with no towing and highway only use with consistant driving above 70 mph, but I rarely run a whole tank without most of it being towing miles, so we will have to wait and see on that

 

[AKaiser]

Today's fill: 18. 2 MPG on 500 miles of 50% stop and go, 50% 60MPH straight thru commute, hand calculated.

 

[BHaden]

Man I love guys like you, your willing to take that chance and separate yourself from the herd. Way to many people on this site and in general only talk about what they would like to do or have done, you are a doer and not a talker, that is refreshing. Don't listen to the nay sayers, if it was up to those people we would still be living in caves and using torches to see. Weather or not the gear change gives you your 2 MPG increase you want, your still a winner, good luck with the swap and keep us posted.

I second this thought. I can't afford anything different right now but have pondered a GearVendor unit when I can, simply because I also run empty a lot and have reached for "7th gear" a few times. Your results will help me make up my mind. Assuming engine calibration is not hugely different (and inefficient) at the lower RPM, less rpm means less friction and more power, percentage-wise, to the wheels. Some of these ideas may not even make total sense dollar/payback-wise, but if they make the truck more enjoyable so what? Thanks for being the guinnea pig for us.

 

[Ereynolds72]

I want to follow up with the comment someone made about the G56 being offered with a higher OD ratio. Is this something that could realistically happen? Is one available in the aftermarket. If they could get that back down to aout . 73 or even . 70, that would solve a lot of problems.

 

[Mrudolph]

I wrote to AAM this morning this was his reply for my truck:



Martin,

There is a 3:42 ratio that is being developed for the 9. 25" front and 11. 5" rear axles. They should be available late May or early June. The 9. 25" is for the front of a 4 wheel drive, but to change the ratio the differential case will have to be changed.



Bob Dunn

Sr. Service Engineer

American Axle & Manufacturing

World Headquarters

One Dauch Drive

Detroit, Michigan 48211-1198



Mail Code 4N-3-38

Phone # 313-758-4400

Fax # 313-758-3784

E-mail -- email address removed --

 

[AKaiser]

I want to follow up with the comment someone made about the G56 being offered with a higher OD ratio. Is this something that could realistically happen? Is one available in the aftermarket. If they could get that back down to aout . 73 or even . 70, that would solve a lot of problems.

Allpar is showing a . 74 on a 08 Chassis and Cab http://www.allpar.com/trucks/chassis-cab.html

 

[MCummings]

... that would solve a lot of problems.

Which problems?



Merrick

 

[AKaiser]

Which problems?



Merrick

The "G56 Top Gear RPMs at Interstate Speeds" Problem

 

[MCummings]

Oh,, then you would mean, percieved problems, and not real problems like transmission overheating, or "5th gear nut" problems, or oil aereating problems.



Thanks for clarifying.



Merrick

 

[AKaiser]

Oh,, then you would mean, percieved problems, and not real problems like transmission overheating, or "5th gear nut" problems, or oil aereating problems.



Thanks for clarifying.



Merrick

Oh Brother. Of course, since I drive the truck every day, I only "perceive" that the engine speeds at 2400 RPMs at Interstate Speed are a problem robbing me of some fuel economy AND wear and tear on the engine.



Of course, you were just saying that to bait me, weren't you? and Ferd owns CumminGs. :-laf

 

[MCummings]

Maybe some baiting, but not entirely on purpose, it just happened.



I just come from the school of run the RPM high and keep the load low on the drivetrain, using the lowest gear possible everything will last longer, and run cooler as long as it was designed properly. It might be at the expense at personal comfort, and fuel, but maybe stress on the drivetrain is lower, but if it stresses you personally more is it really worth it?



That's my train of thought, and why I said what I did. I do feel it is geared a little high for empty driving, but just right for towing anything over 5,000Lbs.



Merrick



Yah, I own Ferd... :-laf

 

[BHaden]

I would like to have the option of lower RPM running empty. If the gearing is too tall for towing; downshift. That's why we have multiple gears in the first place, and as a bonus, since 5th is a direct "through-the-trans", 1:1 with no gears involved in the power transmission, there would be a LOT less heat generated in the trans and 6th gear would not have strain on it. I would think this would be better for the guys who tow really heavy. Gear Vendors edven touts this in their brochure (OK; I realize they are trying to sell something). They say put the G-56 in 5th and engage the OD to keep the main gearbox happier. Of course this negates the use of the E-Brake, but it gives more options.

 

[AKaiser]

Maybe some baiting, but not entirely on purpose, it just happened.



I just come from the school of run the RPM high and keep the load low on the drivetrain, using the lowest gear possible everything will last longer, and run cooler as long as it was designed properly. It might be at the expense at personal comfort, and fuel, but maybe stress on the drivetrain is lower, but if it stresses you personally more is it really worth it?



That's my train of thought, and why I said what I did. I do feel it is geared a little high for empty driving, but just right for towing anything over 5,000Lbs.



Merrick



Yah, I own Ferd... :-laf

I understand what you're saying, but after a 7000 mile trip across the southwestern United States this January (all empty) the darn thing at 75 MPH and 2400 RPM for 8 hours is a tad bit grating. As if that wasn't bad enough, wait until the speed limit is 80 in W. TX Not as bad, noisewise, as my old 99 gasser with the Gibson headers and dual exhaust (somewhat annoying drone. ) Pushing some serious wind at 80+, but 12 MPG??? Now, even with that stretch, my average MPGs for the trip was 16. 8 (not bad, I suppose). Thank God it wasn't a Second Generation truck for noise concerns. Also, thank God for Sirius :-laf If they're not going to put the . 74s in the G56 like the allpar site shows in the C/Cs, I'd go with the 68RE 3. 73s on a new one in a heartbeat, and probably would anyway. If I can get . 74s in this transmission on a replacement upgrade, I'll do that as opposed to messing with the axles.

 

[JWashburn]

Lets analyze this as far as we can. The only thing changing is the r&p.

The wind resistance at 65 (or any speed) is the same. The force required to overcome the wind resistance and other friction is the same for either 3. 73 or 3. 42. Going to the 3. 42 does increase the torque required from the transmission output/engine to increase to overcome the wind resistance/friction forces.

We know or can calculate the horsepower requirements for the 3. 73 because we know the fuel consumption rate. We know that the torque required for the 3. 42 increases. What we don't know is the horsepower need at the lower RPM but greater torque is.

A guess is that one has to depress the go pedal a little further, but that is offset by fewer injection cycles at the lower RPM. I suspect Cummins can easily calculate the outcome, but we aren't privy to that information. It seems the only way is for somebody to do it.

 

[Mrudolph]

Maybe some baiting, but not entirely on purpose, it just happened.



I just come from the school of run the RPM high and keep the load low on the drivetrain, using the lowest gear possible everything will last longer, and run cooler as long as it was designed properly. It might be at the expense at personal comfort, and fuel, but maybe stress on the drivetrain is lower, but if it stresses you personally more is it really worth it?



That's my train of thought, and why I said what I did. I do feel it is geared a little high for empty driving, but just right for towing anything over 5,000Lbs.



Merrick



Yah, I own Ferd... :-laf

My "percieved" issue with this transmission is I haved towed coast to coast and only had to downshift once to pass a line of trucks going uphill, usually I can do this without downshifting due to the low gears and power, as soon as the 3. 42 is available I'll have it. If I then have to downshift great, I doubt if this will harm my engine in the least as my last CTD had higher gear ratios and conciderably less power.

Marty

 

[Turbo_Bob]

1. The wind resistance at 65 (or any speed) is the same.



2. The force required to overcome the wind resistance and other friction is the same for either 3. 73 or 3. 42.



3. Going to the 3. 42 does increase the torque required from the transmission output/engine to increase to overcome the wind resistance/friction forces.



4. A guess is that one has to depress the go pedal a little further

1. Wrong ! What happens when you stick your hand out the window at 55 MPH vs. 80 MPH ?



2. No, the lower geared trans (larger number) will not need as much H/P & T/Q from the engine to overcome wind resistance and friction.



3. Ture.



4. True, if there is not enough H/P & T/Q available at that throttle posistion to overcome the taller gear (smaller number).



Please read this page, it explanes everything.



http://www.pumaracing.co.uk/TOPSPEED.htm

 

[JWashburn]

1. Wrong ! What happens when you stick your hand out the window at 55 MPH vs. 80 MPH ?

The wind resistance is the same for the 3. 73 and the 3. 42. Poorly stated I guess.

2. No, the lower geared trans (larger number) will not need as much H/P & T/Q from the engine to overcome wind resistance and friction.



3. Ture.

I just meant that the friction forces plus the wind resistance are the same for either 3. 73 or 3. 42. The torque required from the engine is greater for the 3. 42.



We can get the dyno charts, but normal driving is somewhere under the curves. I wonder if throttle position is anywhere close to the percentage of the max torque and hp for any given RPM. If so then one might be able to infer an approximate throttle position increase at the new RPM. The concern would be if the new RPM was in the region of the HP curve where it had a steeper positive slope. Anybody got a stock HP curve to post?