Swapping from 3.73 to 3.42 Gears

[DHerbeck]

I might be wasting about 600 bucks, but I am going to try this. Haven't seen anyone at all do it, looks like I will be the guinea pig. If anyone has done it and I missed your post, please chime in.



I have a 2006 QC 2WD, short box Laramie, 27,000 miles, 48RE, 3. 73 limited slip.



I have acquired from AAM the part number for 3. 42 gears for my rear end. They hooked me up with a local distributor and they ordered them for me, about $450. 00 and $200 to install.



I have read a lot of posts about O/D units. This swap will save me 8. 3% on RPM's. That equates to the following:



MPH RPM 3. 73 gears RPM 3. 42 gears

60 1650 1500

65 1800 1650

70 1950 1800



I get 2 MPG increase in MPG slowing 5 MPH in speed. The 3. 42 gears will slow my engine about 150 RPM at 65MPH, which would be like driving 60 MPH. I am hoping to get 2 MPG increase driving 65, turning 1650 RPM, same as if I was driving 60 now. I might not net all the increase, but expect to get close to 2 MPG increase.



Here is the only worry I have... . I pull a 5000 pound enclosed car hauler occasionally, maybe 10% of the time, otherwise I am empty. I have pulled the trailer at 65 1800 RPM, and extended times at 60 1650 RPM and get about 2. 5 MPG better at 60.



These trucks have way plenty HP and torque. I can pull hills at 60 all day without ever unlocking the TC. That tells me that I should not be hurting my occasional towing capacity. I am smart enough to understand that I might be chopping a couple thousand max towing pounds off the top, but I never go there. My engine produces max torque at 1600 RPM and I will be a little above that at 65.



What do you guys think? I am OK with spending about 650 bucks for a solid 2 MPG increase for the life of my truck. I would not go any lower on the gears unless I NEVER towed.

 

[jnelson]

I used to pull 25-32,000 lb gross combined weights with my 01 and it had an auto w/ 3. 55 gears. Pulled like a dream for 430,000 miles. I really dont see a problem towing 5,000lbs.

 

[NIsaacs]

Should be a cool test, however I don't think the mpg will be there. The speed is way more important in fuel mileage than rpm, within reason.





"NICK"

 

[Nelssen]

Taller tires will get you close to that ratio.

 

[Morse]

Do you mind giving me that part number. I got on here early this morning just to try to find some sort of posi or locker (my new truck is an open dif), and see if I could find info on the 3. 42's. I have to crack it open anyway, so I figure I might as well try the 3. 42's while I'm at it. I had the dealer locate me a new leftover 2006 standard cab 2wd that I take delivery of today. I'm cutting the frame, installing a short bed, and transferring all of the performance parts from my dually to this new lighter truck to be a daily driver/play at the track truck. I'm interested to see what kind of mileage I can pull out of the truck. . If you could give me the part number, I'd really appreciate it. . Thanks. .

 

[10.5 Dave]

I doubt you'll see much if any improvement in mileage. Considering the average overall speed for the life of the vehicle should be around 35 mph I don't think the 70-75 rpm decrease in rpm will do much.


My rig has 53,000 miles and my avg speed is 37 mph. I couldn't justify the expense of a R&P swap IMO.

 

[tgrfan2]

I saw the torque curve on a 2006 not long ago. It dropped like a rock below 1600 rpm. Found the same thing on my 06 since it doesn't want to downshift sometimes. You may have to tow a little faster than 60 with the new gears or downshift a little more often on hills loaded.

 

[DHerbeck]

Morse, happy to share the part number with you. Here is what AAM gave me



"Rear



11. 5" Gear Set - 40045439 (New) - (Fits both GM and Chrysler 11. 5 Rears)"



They gave me the name of a few dealers close to me. I called the closest and he was able to order from AAM. Please keep me posted as to your progress as I will do here also. I have a lot of past mileage checks with my truck and run the exact same road all the time, so I should be able to have a good comparison.



Roofer Dave, at 1800 RPM, that gives me 65 MPH with 3. 73 gears. 3. 42 gears are 8. 3% shorter, so at 65 MPH, I should slow the engine to 1650 RPM, the same revs I see at 60 MPH, where I get 2 MPG better mileage. I will in no way be lugging the engine at 1650, so I honestly expect to see most of that 2 MPG gain. I would be happy with that, even a bit less.



Like I said, I might be wasting my time and money, but I am going to try it, 'take one for the team' if you will, as I hear thousands of guys on this forum wanting 3. 42 gears but have seen nobody do it and report results.



Roofer Dave, I just re-read your post and now understand where you are getting 70-75 RPM, using 35 MPH as average. I don't know where you get that number, maybe the ECM stores it, but I do know when I drive highway, I spend 95% of my time at 65 MPH, so I think it will help. Humor me



Dave

 

[TowPro]

be sure to tell us your results when you get them in! More then one AA owner here is wanting to know.

 

[DJennings]

I am interested in the results as well. I don't think you'll see a solid 2mpg either though. It is the wind resistance that hurts more than the rpm.

 

[10.5 Dave]

Roofer Dave, I just re-read your post and now understand where you are getting 70-75 RPM, using 35 MPH as average. I don't know where you get that number, maybe the ECM stores it, but I do know when I drive highway, I spend 95% of my time at 65 MPH, so I think it will help. Humor me

Dave

What I mean is in the 53,000 miles I've put on my truck I've averaged 37 mph combined city/highway(stored in the ecm BTW). At my avg 37 mph I would see a 75 rpm drop going from a 3. 73 to a 3. 42 which would make a miniscule improvement in mileage.

If I drove on the freeway 100% of the time at 65 mph I would see the rpm drop you're talking about but I don't drive at 65 mph all the time so my overall average speed is more imprtant.

I know I'm right maybe I'm not explaining well enough. If you think I'm wrong please humor me and explain your theory. :-laf

 

[DHerbeck]

Roofer Dave,



I am not at all calling you wrong. My point was that when I first read your post talking about average 35 MPH and 70-75 RPM drop, it did not click with me that you basically cut in half both highway spaed and RPM.



I don't expect a 2 MPG average increase for all the driving I do. With the 3. 42 gears, I will for sure save 8. 3% of the revolutions my engine turns to get me everywhere I go. That should save fuel, at some rate, maybe negligible, maybe something I will see in highway hand calculations. I will just have to see.



One thing I do know, guys talk all the time about worse mileage going from 3. 73 to 4. 11, a little more spread than I am doing. They also speak of worse mileage with taller tires that would save the same revolutions I am looking to save, but they blame it on additional rolling mass and friction, never wind resistance or RPM. I am not changing tires, just slowing my engine down 150 RPM at the highway speed I drive 95% of the time, moving it closer to its most efficient RPM of 1600. Now that could be a debate..... does max torque at 1600 RPM mean that is where the engine runs at peak efficiency? I don't know the answer to that. I do know that when I am towing and slow to 55, I am turning about 1600 RPM and I have no lugging, no problem pulling hills, no problem speeding back up to 65, never unlocking the torque converter. I also get about 2. 5 MPG better towing at 60 than I do at 65, about the same difference as unloaded.

 

[10.5 Dave]

Roofer Dave,

I am not at all calling you wrong. My point was that when I first read your post talking about average 35 MPH and 70-75 RPM drop, it did not click with me that you basically cut in half both highway spaed and RPM.

Okay cool. I've done many gear swaps over the yrs and have found it makes little difference in fuel economy.

With the 2wd you have maybe you'll see something. Report back if you make the change.

 

[bighornram]

MPH RPM 3. 73 gears RPM 3. 42 gears

60 1650 1500

65 1800 1650

70 1950 1800



Man I would give an arm and a leg for those RPM's!!!



2100 @ 65 #@$%!

 

[Bertram65]

If you don't tow then you should not have any problems, though I doubt you will see much of MPG increase.

If you do tow it will be harder on the trans all around, lower RPM = lower line presure, not to mention these CR engine are gutless below 2k RPM so towing any kind of hill in OD will be tough.

 

[DPKetchum]

Changing from 3. 73 to 3. 42 is not much change on a auto for $600. 00 bucks. I wouldn't. 4. 10 to 3. 42 for highway driving I would If I knew I wouldn't be pulling much. I've HAD these trucks with 3. 55 gears. What a DOG pulling any weight. These newer trucks have more punch but D. C. wanted AWAY from 3. 55 a LONG time back for a lot of reasons. Now a G56 6 speed I could see 3. 42 but NOT with a auto already geared at 3. 73. Use the money more wisely.

 

[Morse]

There are a lot of variables. My 95 cummins pulled a 12000 pound load very well with the high gears. . My old 99 7. 3 with 4. 10 gears got much worse mpg than the same truck with 3. 73's (my brother inlaws). We'll just try it and see what happens. .

 

[CTieman]

After owning a Smarty for about 2 months now and costing approx what this gear swap would run, I would look into the Smarty. I got a 1-2 mpg increase across the board running on setting 1 (30 hp w/ timming) not to mention the throttle response I gained. I can now read codes also.

 

[Vaughn MacKenzie]

Now that could be a debate..... does max torque at 1600 RPM mean that is where the engine runs at peak efficiency? I don't know the answer to that. I do know that when I am towing and slow to 55, I am turning about 1600 RPM and I have no lugging, no problem pulling hills, no problem speeding back up to 65, never unlocking the torque converter. I also get about 2. 5 MPG better towing at 60 than I do at 65, about the same difference as unloaded.

DHerbeck I would surmise that the "best" RPM to operate at depends on the load you're putting on the engine. Easy cruising with little load 1600 is probably good. Towing or climbing grades then the most efficient RPM is going to be higher. The higher the load the higher the most efficient RPM is going to be. Over the years reading a lot of posts on TDR the RPM numbers best for towing that come up the most are in the 1900-2100 range.



Peak torque may occur at 1600 but you also have about as much torque at say 2200 RPM, so it makes sense to use whatever RPM the engine most easily gets the job done instead of trying to make one certain RPM the answer for the majority of driving.



Vaughn

 

[AKaiser]

Waiting to hear the results of this one. My question is going to be "do they have the gears for the front axle for 4x4s yet?" I've got a G56/3. 73, and 33 inch tires. Most of my driving is highway. Running from Indy to Evansville speeds average 55 if I drive the short route (IN37-US50-US231-I64 aka the proposed I-69 corridor), or 70 if I drive the fast route (I65-I265-I64). I gained 0. 5 MPG roughly, by adding 3/4 inch radius to my tire. I don't want to go any larger and have to lift my truck, etc. My cost to go to new tires and 18" rims was $1500. Sounds like it would cost about as much to put the 3. 42s in. I have the stock rims, so I could always put 245s on them and tow if necessary. They have a fairly good set of 265/70R17 BFGs on now in the shed.