Swapping from 3.73 to 3.42 Gears

[Morse]

For a standard transmission, I think the gear swap would be great. . If you don't pull heavy loads with the auto, or seldom pull it would be a good choice as well. . If you pull heavy with an auto, I just don't think it would be the best decision unless you locked out OD. With the help of the 3. 42's, my 06 ran 119. 69 mph in the quarter at just under 3300 rpms this past weekend (28. 8'' drag radials). .

 

[josh8811]

you'll also have a truck with a top speed of over 120MPH. .

 

[seafish]

You know, after driving on the highway unloaded for all day yesterday at 67mph at 2200 rpm in 6th gear, I am seriously thinking about doing this mod sooner than later. Anyone here have a total cost invoice for the parts, and also opinions on how involved it would be to swap out the diff gears for a reasonably mechanically inclined person with a decent shop space, air tools etc. ?

 

[Dbonzi]

Hey you guys how about the milage?

Everyone said they put on so many miles it would pay for itself in X amount of miles.

Where are the reports?

Dennis

 

[Dbonzi]

Well?? how are the gears treating you?

Milage?

Bueller... anyone???

 

[courierdog]

3.73 to 3.42

I am waiting for my dealer to give me the final pricing of the change over.

The new 2008 models have the 3. 42 as an option, with the caution of no pulling or hauling. As 90% of my driving is at 60 MPH running less than 500 lbs load it should net me about 2 MPG. I run a nominal 6000 miles a month and I would like to keep it for 500K if the rest of the truck holds together that long.

As I run Hot Shot/Courier service fuel economy and reliability are very important issues.

I spoke with AAM and they passed me off to a local vendor.

My dealer is willing to do the work and is waiting back on confirmation of pricing.

I have friends who have done this in the past with the old style rear end and the results were great in the milage department. It all depends on what you use your truck for, if you are pulling or hauling any load. If you are running light like myself then even going down to 3. 21 is viable however you do have to understand the trade offs. Changing out the Rear Axle Ratios is not for everyone.

 

[Turbo_Bob]

Also going from 18 to 21. 6 mpg is actually a 20% increase in mileage

You took 18 X 20 % = 3. 6 + = 21. 6



I have allways been told to get percentage you need to go backwards.

21. 6 - 16. 66 % = 18. 00



So it is either 20 % or 16. 66 % ?

 

[Cattletrkr]

You took 18 X 20 % = 3. 6 + = 21. 6

I have allways been told to get percentage you need to go backwards.
21. 6 - 16. 66 % = 18. 00

So it is either 20 % or 16. 66 % ?

Not to get too off track, but I think he's talking about increasing the mileage from 18 to 21. 6. That would be a 20% increase from the original 18mpg. Now if we're talking losing mileage and we're starting out at 21. 6, then it would be a 16. 67% decrease.

 

[raychem]

I have been running my gears for a few months now... . a 2 mpg increase is very optimistic. I have been getting an average 8/10 or . 8 mpg improvment in overall mpg with the gears. NOW when I installed the cam in the truck I gained a full 1 mpg not to mention that my turbo spools much quicker and egts are down about 125-175 * overall. I must admit though that my cruising rpm has dropped from 2500 @ 85 to 2175@ 85 so that alone to me warrants the gear change. The engine is not reving as high at highway speeds. Like with anything else... . even with the gear change, speed will be the last determining factor in fuel economy improvment.





Richard

 

[courierdog]

change

You mention you also have changed your cam. From what to what.

 

[raychem]

You mention you also have changed your cam. From what to what.

From stock to a max spool cam

 

[JChamberlain]

So is there anyone with a G56 that has done the swap?

 

[courierdog]

I finally have the 3. 42 gears in the truck.

It sure makes a big difference in how the truck behaves.

There is still lots of bottom end torque left so there is no problems with acceleration when required.

Makes the truck more quiet at speed.

I have not had them installed long enough to have a drive where I can measure the fuel economy difference.

I will post and update when I have those results.

 

[DPrew]

There no truck info on your signature is your truck stone stock? If it is an exhaust, and air box might be a better investment. A FASS system also might be a better investment. FASS is supposed to be good for a 6% increase in milage and has other benefits as well. I love my FASS 95.

 

[DPrew]

Wow! I didn't see all 15 pages of replies. Something nobody seems to be analyzing is that RPM's and MPH will not rise in perfect correlation to each other from 0 - 75+ MPH. At 70 MPH the truck is going to have three times the friction resistance that it does at 30 MPH. Not twice the resistance most people expect. Once you get above about 55 MPH you should see the RMP's start to increase more quickly than MPH as a percentage.



I'm spoiled on driving 75 MPH but if the U. S. reduced the speed limits back to the max limit of 55 MPH I be very interested to see what effect it would have on total U. S. fuel consumption. I'm sure the numbers would easily add up to be into the billions of gallons.

 

[courierdog]

Just finished a days driving and confirmed the following actual and calculated.
Per the On line Calculator assuming the 68RFE and 3. 42 Rear Axle
1100 RPM 48 MPH
1200 RPM 53 MPH
1300 RPM 57 MPH
1400 RPM 62 MPH
1500 RPM 66 MPH
1600 RPM 70 MPH
1700 RPM 75 MPH
1800 RPM 79 MPH
1900 RPM 84 MPH
I still require a single trip of significant milage to compare actual fuel efficiency.

 

[courierdog]

There is a caution:
When running around town at speeds less than 50 MPH I suggest you run in Tow / Haul mode in 4th otherwise you will be going into active regeneration more often and for longer periods when you do reach speeds greater than 50 MPH. Plus the milage will also go down around town in gears higher than 4th.
If you have the 3. 42 gear set try it and see what I mean.

 

[DZinger]

Waiting (patiently???)

All - I, too, and anxious to get some real results. Like you, AKaiser, I am in Iraq as well waiting to go home (buying goodies up while here). I am running the G56 as well and am looking to see if the swap is worth it.



Another possible source for the gearset appeared lastest DP mag - Differential parts and accessories | Ring & Pinions | J. T. 's Parts and Accessories



In the mean time, I wait... . but not too patiently... .

 

[courierdog]

Well I must say at this point there must be something terribly wrong as my milage actually went down. On repeated trips the milage has decreased by almost 2 MPG rather than increased. The truck seems to be running happier at the lower RPM's but the fuel economy tells a different story. It seems like the ECM figures any running around the city is excessive idling and goes into regen as soon as there is any sustained highway speeds. Also we have been hit with a bad cold spell and lots of snow and the milage also has taken a tumble, never had a diesel do this before.

 

[Bob4x4]

sorry if you already covered this but did you change the ECM programming to reflect the 3. 42 gears?

No change is needed with this year truck,the tire size will change it but not the gear ratio.



Bob