Swapping from 3.73 to 3.42 Gears

[Forrest Nearing]

yeah, a 6sp w/ 3. 42's front/rear would be a pretty sweet rig!

 

[AKaiser]

3.42 geared G56

Yes

• 3. 42 gears, 33" tires, 1st gear, 1000 RPM = 5 MPH
  
  .
  
• 3. 73 gears, 33" tires, 2d gear, 1000 RPM = 8 MPH, which is the same as it is at 3. 42s
  
• I can still take off in 2d Oo.
  
• 3. 73 gears, Stock 265/70R17s, 2d gear, 1000 = 7MPH
  
  .
  
• If I'm driving between 45 and 60 MPH, 5th gear would be optimal
  
  .
  
• Sixth would get used pretty much only on the interstate
  
  .
  
• Seventy MPH in 6th is well within the power band for towing at just a shade over 1900 RPMs
  
  .
  
• If I feel I need an extra bit of RPMS, I could always swap out my 33s for the stock tires/rims for an extra 100 RPM at 70
  
  .
  
• Going to 245/70R17 would get it up to 2100 at 70
  

 

[MCummings]

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.

With your reasoning, I could travel 150MPH at 1,650RPM and get the same mpg.



Wind resistance, and the power required to push your truck through the air is a bigger factor than RPM.



Lowering your truck, running lower profile tires, and smoothing out the undercarriage, along with running a camper that slopes from the cab, to the tailgate would make a bigger difference.





Merrick

 

[Ereynolds72]

Would you have to reprogram the PCM to accommodate the higher gear ratio? It seems, just like putting bigger tires on, that this would screw up the computer. Could the dealer do this?

 

[DHerbeck]

With your reasoning, I could travel 150MPH at 1,650RPM and get the same mpg.



Wind resistance, and the power required to push your truck through the air is a bigger factor than RPM.



Lowering your truck, running lower profile tires, and smoothing out the undercarriage, along with running a camper that slopes from the cab, to the tailgate would make a bigger difference.





Merrick

Merrick,



My reasoning is only concerned with slowing my engine 150 RPM at 65 MPH, to what is now turns at 59 MPH where I get more than 2 MPG increase. I know some wind resistance is in play, but no more then 5-6 MPH. That is the difference in no headwind and a 5-6 MPH head wind which does not hurt me too much. I can drive 70 MPH with 20-25 MPH sustained headwind, enough to make the door seals sing to me and only lose about 1-2 MPG. That said, I do not think the 5-6 MPH wind difference will wash out ANY fuel savings.



If RPM has nothing to do with it, someone with a G56 humor me with this request:



Drive 65 MPH if 5th gear and tell me your RPM. Shift to 6th and tell me your RPM at the same 65 MPH. Now, fill up, drive 100 miles in 5th gear, fill up and hand calculate MPG, then drive back the same 100 miles in 6th gear, fill up, hand calculate MPG. . If wind resistance is the only contributing factor, not engine RPM, your MPG figures will be the same. I honestly do not believe they will. I am certain there is a 6th gear there for highway mileage gains, slowing the RPM's at any given speed.



Again, I am only talking about 65 MPH and slowing the engine by 150 RPM and looking for something in the range of a 2 MPG increase. I am in no way implying that I can more than double my speed to 150 MPH and get the same MPG. I don't know where you ever read that implication in my post.



I am attempting to do something that hundreds of guys on this board have wished for, just didn't have the information to get it done. The only posts I have read about 3. 42 gears and AAM are that they are not available. Well, according to AAM, they are. I have found that information, shared it with you guys and actually expected interested support. Maybe I haven't been around here long enough to get any respect, just criticism, I don't know.



Don't get me wrong, I am not upset, just expected more support than what I am seeing. Maybe I should not even be posting here. Maybe I should just get it done, report a 2 MPG gain and be called a liar, I don't know.



And trust me, when I get this done and if I see NO MPG increase, you can all say "Told you so!".



Dave

 

[Turbo Tim 1]

Hey don't get upset, some people just like to be nay-sayers, myself I'm watching this thread to see what happens and also to see if they are making them for the front axle also. Keep us informed.

 

[DPKetchum]

Curious? What did you buy a diesel truck for? I bought mine to tow heavy with. Fuel mileage was not the first reason. I've had a 160 H. P. with 3. 55s. Got a FEW mpg better and towed like it was chained to the Titanic. I can see a G56 transmission truck with 3. 42s. But that the only one in the D. C. diesel line up with such. Just my thoughts. You can run too little rpm and to cool. I never thought HUNDEREDS wanted a 3. 42 just some G56 owners and some owners that wanted the truck for reasons out of its design. 3. 73 ought to be a good compromise gear for a auto truck for moderate towing and moderate fuel mileage.

 

[JMarketos]

Looks like the core focus is on efficiency and not specifically

on engine RPM. Lowering engine RPM does lower some

internal engine losses (friction), but may get you out of the

engine's "sweet spot", then efficiency would suffer.



Think twice about crimping the cruising RPM's down, especially

with an auto trans that won't allow you to hold it in 3rd gear.

I also tow a fairly light trailer and often find, as others seem to,

that the aero drag of the trailer comes into play just as much

as the weight of the payload.



Another thing I haven't seen anybody mention yet-

I have both pre and post-turbo EGT's installed. If you don't have

an exhaust temp gauge installed before doing the gear change,

I'd strongly recommend get one. I think you'll be surprised

(perhaps alarmed) at the EGT readings you see while running

the engine at 1400-1500 RPM (and less) under extended load.



I really liked the suggestion in another reply about trying larger tires

as a means to simulate the stiffer rear gears. Maybe you could

even make some connections and borrow a pair for a test run.



Good luck !

 

[DHerbeck]

DPKetchum,



You have some very good points. I bought a diesel truck to tow 5000 pounds with about 3000 miles a year. The rest of the 22,000 miles is with no load. I have plenty of friends who have gas Dodge trucks and the diesels get more than 50% better fuel economy. The time I tow empty, I would like to get the best mileage I can.



This board is full of products and ways that guys are trying to get better mileage. The way I am trying to is yet another way that nobody has done before.



This might be a poor example but here goes... . a new Corvette is a very fast and powerful vehicle and sucks fuel quite rapidly when driven hard, but can get 32+ MPG on the highway due to gearing in top gear.



You guys might be right. I may be totally fooled about RPM's and fuel economy, EGT's, etc. I just know that when I slow down, I get dramatically better fuel economy and it all can't be attributed to wind resistance.



Doing the 3. 42 gear swap might render my truck useless for towing. As someone else suggested, one could put smaller diameter tires on to bring it right back to the gearing it has right now for the 10% towing I do, which is all at one time.

 

[RMachida]

Plus you could always tow in 5th if turns out to be an issue. I agree that the G56 trucks are probably geared too low when running empty. I don't know how many times I started looking for another gear. Looking forward to your results.

 

[DPKetchum]

See just to tow 5000 lbs I would NOT have spent all the extra coin for a any of the big three diesels. A nice 1500 these days will do THAT(5k) with no issues.

 

[MCummings]

Well Dave, just because I wouldn't do it, doesn't mean I should bring you down now does it. Good luck, and let us know.





The ECM/PCM does not have to be reprogrammed for gear changes. The speed sensor is in the differential and only measures axle speed, so changing transmissions, or going to 2low or 4hi, or going from 3. 23's to 5. 11's won't effect your speedo, but changing tires would.





Merrick

 

[CBari]

I May Be Completly Wrong But I Think He Will See The Results He Is Talking About, And Here Is Why.





I Used To Own 3 Different Chevy 6. 5 (before I Bought A Real Engine And Truck To Go With It). 2 Where 2500 4x4 Auto's And The 3 Was A 1500 4x4 Auto. The 1500 Had 3. 42 Pulling The Exact Same Load It Would Get 2mpg Better Than The 2500 That Had 3. 73 And That 2500 Would Get 2mpg Better Than The Other 2500 With 4. 10's And The Exact Same Load. The Load In Question Was A 3horse Alum. Gooseneck Trailer With The Same 3 Horses On The Same Exact Trip. All Three Truck Were 95 Models By The Way.



Now Would The 3. 42's Pull The Hills As Well No But They Did Get The Job Done And Were Great Empty.



I Wish You Luck And That All Goes Well In Changing Them Out.



By The Way The Trailer Weighed 3500 Empty Plus About 1000lbs Per Horse For A Total Of 6500lbs

 

[Morse]

I'm laughing at some of the negative comments when they have no experience trying this combo. This is something all of us are interested in seeing how it turns out. Let's support him in his venture. Why wouldn't someone spend an extra 5 grand on a truck, even if you don't tow at all? I would. Between the fuel savings and the trade in value it's worth it. I just picked up my new leftover 06 2500 standard cab Sunday. I may be chopping the frame this weekend, and installing the short bed. I don't plan to tow much with it. . haha. . I ordered a set of 3. 42's and a new posi from Greensburg Machine today for a total of $850 for both. Very good guy to deal with.

 

[DPKetchum]

4. 10 to 3. 42 I can see. 3. 73 to 3. 42 is a waste of money. But whatever floats you.

 

[Morse]

That's possible. . It would take around 25,000 miles to pay for the upgrade (cost of the gears alone) if mileage increases 2 mpg. Most any upgrade for our trucks could be considered a waste to some. They are fine completely stock. The good thing is, we don't have to keep them that way.

 

[DHerbeck]

Well Dave, just because I wouldn't do it, doesn't mean I should bring you down now does it. Good luck, and let us know.





The ECM/PCM does not have to be reprogrammed for gear changes. The speed sensor is in the differential and only measures axle speed, so changing transmissions, or going to 2low or 4hi, or going from 3. 23's to 5. 11's won't effect your speedo, but changing tires would.





Merrick

Merrick,



Thanks for the above info. I was looking through posts to figure out how to correct the speedo.



I have a few gauges ordered and will get them on before embarking on a 2500 mile towing trip in late April. Will get baseline numbers from that trip, then install the gears and compare/watch things like EGT's and transmission temp.



I just have to think, until proven wrong, that the transmission will hold up to this. If slowing the engine to 1650 at 65MPH will overheat the transmission or cause EGT's to rise, what is a guy to do with a stock truck if traffic only permits driving say 55 or 60, placing him in the same RPM range? I see no warnings in the manual stating to drive above 60 MPH on the highway. I would also think towing a trailer weighing 10000 pounds versus a trailer weighing 5000 pounds would place much more strain on the transmission than slowing down 5 MPH.



I could be all wrong. If I had it to do all over again and was going to purchase a pickup truck just for pleasure driving, no towing, I would choose the same truck... for the Cummins Turbo Diesel. I am sold on these things.

 

[Ross]

I think you'll be happy with the swap. I look forward to your results. About 85% of my mileage is towing heavy (15,000 pound trailer), and have a lot of experience with both 4. 10 and 3. 55 (2nd gens). I actually prefer the 3. 55s. I now have a 2006 mega-dually with the G-56 and 3. 73s, and it REALLY needs another gear.



Frankly, I would ignore some of the above posters that seem to know better than you how to spend your money.



Thanks for posting, and let us know how it turns out.

 

[JMarketos]

I just know that when I slow down, I get dramatically better fuel economy and it all can't be attributed to wind resistance.

I understand the wonder, given the above observation. More than one

thing changed by slowing down; engine speed slowed down and wind speed

over the body slowed down. Was the improved fuel economy due to one

or the other, or combination of both ?



Don't underestimate the contribution of wind resistance.

Aerodynamic drag is a 2nd or 3rd order (I forget) (mathematic) quantity.

That is, for a given increase of one unit in speed, the drag increases

2 or 3 times. Slowing down just a little results in big reduction

in horsepower required to move the vehicle. The faster it goes,

the more (multiple) horsepower is required. That's why the land speed

record vehicles have such hellish horsepower engines.



I may be just re-cycling other info at this point, but I've read elsewhere

on this forum that the new common rail engines run most efficiently

in the 1900 RPM range (while the previous engines were apparently

most efficient at a few hundred RPMs lower).



My prediction is that the gear change may not achieve much better

fuel economy when driving the same speed, but the EGTs when towing

the trailer may be of concern.



The 6-speed guys have their cake and eat it, too !

 

[Morse]

This will be interesting. . I'm holding on to my 3. 73's just in case it doesn't work out the way I hope. . If it doesn't work, then atleast we can pass on the information on so others don't make the same mistake.