Swapping from 3.73 to 3.42 Gears

[AKaiser]

I am definately looking into this. One of my freinds is the head cummins service tech at a dodge dealer here. I have asked him about swapping the taller overdrive from the 4500/5500 G56, but he hasn't been to any class on it yet. I will follow up with him in a few weeks.

You have my attention.

It would be nice if DC replaced my transmission under warranty(5th and 6th gear buzz) with a G56 for the 4500. Unfortunatley they say the gear buzz is normal.

Yeah, whatever. No buzz in mine. I think it's more like, "Some gear buzz is acceptable. " Normal it is not.

 

[CookD]

Real rpm vs mpg test?

What we need is someone to chime in that has a gearvenders or other overdrive installed. That way we could see what the mpg should be at a lower rpm.

 

[BHaden]

What we need is someone to chime in that has a gearvenders or other overdrive installed. That way we could see what the mpg should be at a lower rpm.

Yeah, I may go the GV route for the extra flexibility it gives. I bought my MC for towing but at present I do a lot of unloaded driving and I'm sure it would help. I too find myself going for 7th gear sometimes!



Something else I don't see mentioned a lot is the decrease in engine wear at lower rpm. Not that the Cummins is short-lived anyway, but every bit helps.

 

[jmccart]

Gear vendors are so radical. I will probably never afford one, but they are the ultimate.

 

[DHerbeck]

More thoughts on wind resistance...

Guys,



I just made my monthly 500 mile run. North/South highway, 250 miles one way. Overhead mileage is always 2-4 MPG low, but a good tool to check mileage differences.



Driving south, sustained 20 MPH headwind, constant 65 MPH, overhead read solid 20. 5 MPG. Coming home, sustained 20 MPH tailwind. So, the wind resistance difference in driving 65 MPH into a 20 MPH headwind, versus driving 65 MPH with a sustained 20 MPH tailwind is a 40 MPH wind differential, agree? How much MPG difference do the guys that think it is ALL wind resistance think I observed?



2 MPG more with tailwind, 40 MPH less wind on the front of the truck. Does wind make a difference? Sure it does. But as you can see, 40 MPH wind difference made 2 MPG difference.



I tell you all that to say this..... when I drive 65 MPH and slow to 60 MPH, I see a 2 MPG increase..... with only 5 MPH less wind. If I speed up, I see 2 MPG less MPG per 5 MPH speed increase. I drove 75 MPH with this 20 MPH tailwind and observed a 4 MPG decrease... . that is only 10 MPH more wind than at 65.



So whats the difference? 150 RPM per 5 MPH.



With the 20 MPH sustained tailwind, 65 MPH, 1800 RPM gave 22. 5 MPG. Speeding up to 75 MPH, increasing RPM by 300 to 2100 gave 18. 5 MPG, 4 MPG less. This is observed by resetting the overhead and driving 20 miles to get a good value.



All that said, I really think I will get close to 2 MPG increase at 65 between my 3. 73's and new 3. 42's. I have a 2500 mile towing trip in late April. I will not change my gears until after that trip.



Today's trip sealed my mind about just how much lower MPG is caused by increased wind resistance versus increased RPM. I think probably 25% of the decrease is due to wind resistance and 75% is due to increased RPM.



Whatcha think?



Dave

 

[AKaiser]

Guys,



I just made my monthly 500 mile run. North/South highway, 250 miles one way. Overhead mileage is always 2-4 MPG low, but a good tool to check mileage differences.



Driving south, sustained 20 MPH headwind, constant 65 MPH, overhead read solid 20. 5 MPG. Coming home, sustained 20 MPH tailwind. So, the wind resistance difference in driving 65 MPH into a 20 MPH headwind, versus driving 65 MPH with a sustained 20 MPH tailwind is a 40 MPH wind differential, agree? How much MPG difference do the guys that think it is ALL wind resistance think I observed?



2 MPG more with tailwind, 40 MPH less wind on the front of the truck. Does wind make a difference? Sure it does. But as you can see, 40 MPH wind difference made 2 MPG difference.



I tell you all that to say this..... when I drive 65 MPH and slow to 60 MPH, I see a 2 MPG increase..... with only 5 MPH less wind. If I speed up, I see 2 MPG less MPG per 5 MPH speed increase. I drove 75 MPH with this 20 MPH tailwind and observed a 4 MPG decrease... . that is only 10 MPH more wind than at 65.



So whats the difference? 150 RPM per 5 MPH.



With the 20 MPH sustained tailwind, 65 MPH, 1800 RPM gave 22. 5 MPG. Speeding up to 75 MPH, increasing RPM by 300 to 2100 gave 18. 5 MPG, 4 MPG less. This is observed by resetting the overhead and driving 20 miles to get a good value.



All that said, I really think I will get close to 2 MPG increase at 65 between my 3. 73's and new 3. 42's. I have a 2500 mile towing trip in late April. I will not change my gears until after that trip.



Today's trip sealed my mind about just how much lower MPG is caused by increased wind resistance versus increased RPM. I think probably 25% of the decrease is due to wind resistance and 75% is due to increased RPM.



Whatcha think?



Dave

I think you just proved what everyone else knows about the overhead: It's inaccurate. The overhead "Lie-O_Meter" numbers are based on computer parameters, of which "MET" (meterological data in Artillery terms) is not one of them. To put it in Artillery terms, the computer assumes "standard MET" wind speed, which is 1:1 against your ground speed. If the truck had a Pitot Tube out the nose, like an aircraft has on it's wing, it could perhaps calculate a differential calculation for wind resistance. In reality, it has no clue.

 

[Turbo_Bob]

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

65 1800 1650

70 1950 1800

The sweet spot is 1650 for the start of peak torque. So 65 MPH will be a good speed for pulling the trailer and up to 70 solo.



I have found with my truck 1650, best MPG. And keep the overall speed below 70 for good MPG.

I am in the same boat as you, and been thinking about 19. 5's, overdrive units. I can only go about 1" taller tires because the truck is only 2WD, so I dont think there would be enough improvement in MPG to offset the extra drag & weight of the tires. The overdrive units cost the bucks and I dont really want to modify to that extent. But the 3. 42's I think are what we need too.



The reason why Dodge used the 3. 73's is to be able to pull more weight than the older trucks with the 3. 55's. So going with the 3. 42's, you will not be able to max out to the 14,000 plus pound trailer.

 

[Morse]

If my gears come in soon, I'll be pulling over 12,000 pounds from KY to FL with them in a week and a half. . This should give it a little test.

 

[DHerbeck]

Awesome Morse. I was just a little too scared to put mine in and head to CA and find out i had done the wrong thing. Keep us posted.

 

[DGoyer]

I just had 315/70 17 put on and with the 373 I put on I am close. still don't use 1st empty, but at 48 mph 5th is at 1700 use 6th less around town. have to re flash spedo now.

 

[Turbo_Bob]

here is the info from AAM:

To convert Chrysler (Dodge Ram) to 3. 42 ratio:

Rear

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

Do you have there web site link (AAM) ?

I would like to see about ordering the R&P and find a dealer near me.

 

[Bertram65]

The sweet spot is 1650 for the start of peak torque. So 65 MPH will be a good speed for pulling the trailer and up to 70 solo.



I have found with my truck 1650, best MPG. And keep the overall speed below 70 for good MPG.

I am in the same boat as you, and been thinking about 19. 5's, overdrive units. I can only go about 1" taller tires because the truck is only 2WD, so I dont think there would be enough improvement in MPG to offset the extra drag & weight of the tires. The overdrive units cost the bucks and I dont really want to modify to that extent. But the 3. 42's I think are what we need too.



The reason why Dodge used the 3. 73's is to be able to pull more weight than the older trucks with the 3. 55's. So going with the 3. 42's, you will not be able to max out to the 14,000 plus pound trailer.

Have you ever tried to tow anything at 1650 RPMs? I know my truck is gutless at anything below 2000 RPMs when towing not to mention the EGTs get pretty high trying to pull at low RPMs.

 

[tgrfan2]

Have you ever tried to tow anything at 1650 RPMs? I know my truck is gutless at anything below 2000 RPMs when towing not to mention the EGTs get pretty high trying to pull at low RPMs.

My friend's recently stolen 2003 HO 48re 3. 73 had plenty of guts at 1650rpm GCW 21K. Now it wouldn't get down with my 12V but the 2006 is worse.

I did a little test towing my fiver this weekend due to rain slowing me down on the return trip! Going the 140 miles with no wind and 74 degree temps, I got 10. 1 mpg running at 2000-2100 rpm(70-75mph). Coming back in rain at 64 degrees and still no wind I got 12. 2 mpg at 1750-1800 rpm(65mph). I have to admit that is the most variation in mileage I have ever gotten with my fiver and the best mileage and slowest 140 miles too! I was running 50% LSD and 50% B20 so about a B10 brew. With diesel back to about $2. 60/gal. , I may slow down and smell the B20!

 

[Turbo_Bob]

Have you ever tried to tow anything at 1650 RPMs? I know my truck is gutless at anything below 2000 RPMs when towing not to mention the EGTs get pretty high trying to pull at low RPMs.

I tow around 1900 RPM's, around 60 MPH (6000lbs). I have towed at the 1650 range and on level highways it will work. Going up steep grades it is better to get up in the H/P area as you say in the 2000 area.



I am still not convinced that the 3. 42 R&P will help out with MPG towing something around 6000 pounds and definitely not something 10000-14000 pounds. But I know it will help out when driving solo unloaded.

 

[AKaiser]

I tow around 1900 RPM's, around 60 MPH (6000lbs). I have towed at the 1650 range and on level highways it will work. Going up steep grades it is better to get up in the H/P area as you say in the 2000 area.



I am still not convinced that the 3. 42 R&P will help out with MPG towing something around 6000 pounds and definitely not something 10000-14000 pounds. But I know it will help out when driving solo unloaded.

Probably no point in worrying too much about towing MPG, unless it's just ridiculously low. If I was going to tow on the high end (above 10K) I'd have bought a 3500 DRW, and expect lower MPG. No plans to tow 14K with my 2500, and to this point, it's just a day driver. 10 K with 3. 42s in Indiana should be no problem. Even in the southern hills, there's nothing this rig couldn't haul with 3. 42s that the truck is rated for.

 

[timroberts]

Maybe 1650 rpm not to low using tst box or smarty ;lots of timming help with egt . I like to tow 70 /75 mph 8000 lbs camper on long trips. Just a thought. I also have 3. 73s

 

[DHerbeck]

You guys make some good points. I only tow 5000 lbs max. I have been in the Smokies, (I-40, eastern TN, western NC) 6% grade, pulled like a freight train, even when traffic and curves kept speed down and RPM's as low as 1500, never unlocked the torque converter.



By the way, does anyone know what the big rigs run for RPM at speed? I know their engines are bigger, but there has to be some common ground, we have a truly remarkable heavy duty engine ourselves.



I thought about the GV unit, but inline, that would gear me about 22% lower, more like going from my 3. 73's to something like 2. 55's, way too low. I honestly think the moderate 8. 3% reduction in RPM's will give me the MPG increase I am looking for and leave my truck fully capable of towing anything I will ever need to tow.

 

[raychem]

Where to buy these gears. I have called a few people and AAM said that they were not released for them to sell yet. Go figure, so if you guys can point me in the right direction I would appreciate it.



Richard

 

[Morse]

Hey guys, I won't be able to do any comparisons for a while. . I just talked to Greensburg Machine, and they told me AAM has them on backorder, and don't plan to ship any more out until May 2nd.

 

[DHerbeck]

Contacting AAM

Guys,



Due to the interest, I emailed AAM back to ensure they were OK with my posting their contact information. Here is their reply:



Dave,



Please have them contact me



Bill Laughman

American Axle & Manufacturing

Sales Account Manager

-- email address removed --

Tel: 313. 758. 4598

Fax: 313. 758. 3826