Swapping from 3.73 to 3.42 Gears

[DPKetchum]

D. C. wanted the 3. 55 gone at least from 99 or so a when I asked same questions back then and a rep told me that SURVEY SAYS that 3. 55 not good work gear in majority of situations and the CUMMINS liked 3. 73 or 4. 10s

 

[Matt400]

Dave,

I think you will find 3. 42's are good.

As an example my son put 35's on a 3. 73 auto with . 69 ratio OD.

The mpg's and power did go down some in day to day start / stop mountain driving and the biggest trouble we noted from that was OD could not be used as much because speeds in our area are allot of 50-60 mph stuff, it was just too tall for that kind of driving.



With the 32. 8" tires it did have, you could get locked in OD and just poke along at 50-60 with a low RPM getting much better mpg's



We originally thought the rolling mass was the trouble and that may have contributed but on a coast to coast road trip, 3 people and 600 lbs. in the bed I hand calc 20 mpg at 75 mph with those big 35's and I think that was because we were only at 1850 RPM so the area will play a big roll but with your stick and running empty I can only see an improvement.



I pull 6000 lbs pretty easy with 32. 8" tires @ 1700 rpm, how tall are the tires you use?

 

[Sean Flanagan]

Regarding the 3:55 gears on the 2nd gens. Back when I got my '98. 5, everybody on this board was whining that the gears were too high and wanted to be just like the Ford guys with 3:73. Back then everybody was complaining that the Stock Fords would out-pull the Stock Cummins and blamed it on the gears.



Now everybody wants the 3:55's back.



Kind of amusing to me.



Personally, I preferred the 3:55's on my old 98. 5 over the new '07, but one should look at the pay-off to see if it makes sense.



These numbers are close to my situation.

Annual mileage: 30000

Average mpg: 18 (This is my new '07 with 3:73 gears and 295-70-17 tires)

Improved economy: 20 (That was my old '98. 5 with 3:54 gears and 285-70-16 ties)

Average fuel cost: $3. 15



Before gear change annual cost for fuel $5350

After gear change annual cost for fuel might be: $4725

Annual fuel savings: $525. 00



That looks like a 3-4 year pay-off for the gear change and a 6 year pay-off for an overdrive.



I kept my old '98. 5 for 9 years and 270k miles before the mighty Cummins decided to chuck the crank tonewheel through the side of the block. This gear change thing might just make sense, IF the improved mileage is indeed true. Anybody have any real, no BS numbers?

 

[TowPro]

Still waiting for DHerbeck to put his in.

 

[JHardwick]

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.

You'll be lucky to see 1mpg increase. Wind resistance is the big killer.

 

[birddog]

do you have a formula for your calculations, or do you know what those rpms would be for guys with the g56's.

 

[AKaiser]

do you have a formula for your calculations, or do you know what those rpms would be for guys with the g56's.

http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls



or



http://www.klenger.net/dodge/general-reference/gear-speed-calculator.pdf

 

[RCC]

Does anyone know if the 3. 42 gearsets have become available for sale yet?I have been trying to buy them for over a month and no luck,any ideas?

 

[DHerbeck]

It's been a while since I chimed in here. I stopped by my gear shop last week and AAM still has a backorder on the gears. I am beginning to wonder if they are even available at this point. I am going to contact my AAM fellow again to see what he might say. I am still going to jump on this gear swap as soon as I can get the gears.



Based on the computations a few posts back, this mod will cost me about 650. 00 to complete, not much over a years payout. Not bad when you consider I am keeping my truck for the long haul. That is based on a 2 MPG increase. Even at a 1 MPG solid hand calculated increase, that's not bad.



I am very pleased with a couple very inexpensive mods to date. The Rokktech timing sensor set to full advance and the Rokktech straight thru muffler. About 150. 00 worth of mods good for 2 MPG consistent increase. I also feel that topping 25,000 miles was a nice sweet breakin period for this engine. I have done nothing to the intake, nor have I switched to anything synthetic yet.



My daily drive to work is a 15 mile one way, 10 highway and 5 city commute. I consistently average 17. 5 to 19 MPG per tank hand calculated. Think of the number of cold starts across 2 weeks on a tank! Sustained highway at 65 is always above 22, as high as 26. 7 at 60 MPH on cruise for 250 miles. That was sitting on 1600 RPM.



I will keep you posted on what I hear from AAM on the gear availability.



Dave

 

[amsoilman]

It's been a while since I chimed in here. I stopped by my gear shop last week and AAM still has a backorder on the gears. I am beginning to wonder if they are even available at this point. I am going to contact my AAM fellow again to see what he might say. I am still going to jump on this gear swap as soon as I can get the gears.



Based on the computations a few posts back, this mod will cost me about 650. 00 to complete, not much over a years payout. Not bad when you consider I am keeping my truck for the long haul. That is based on a 2 MPG increase. Even at a 1 MPG solid hand calculated increase, that's not bad.



I am very pleased with a couple very inexpensive mods to date. The Rokktech timing sensor set to full advance and the Rokktech straight thru muffler. About 150. 00 worth of mods good for 2 MPG consistent increase. I also feel that topping 25,000 miles was a nice sweet breakin period for this engine. I have done nothing to the intake, nor have I switched to anything synthetic yet.



My daily drive to work is a 15 mile one way, 10 highway and 5 city commute. I consistently average 17. 5 to 19 MPG per tank hand calculated. Think of the number of cold starts across 2 weeks on a tank! Sustained highway at 65 is always above 22, as high as 26. 7 at 60 MPH on cruise for 250 miles. That was sitting on 1600 RPM.



I will keep you posted on what I hear from AAM on the gear availability.



Dave

I just read an interesting article on Fuel Economy. According to Roger Clark, senior manager for General Motors Energy Integration and fuel economy learning Vehicles Program (ELVP), fuel economy doesn't change much with an optional gear ratio. "The typical combined fuel-economy impact, based on 0. 4 MPG to 0. 6 MPG between the base gear ratio (e. g. 3. 42:1) and the lowest (e. g. 4. 10:1) offered," says clark. That change is linear. Equipping a truck or SUV with a 3. 73 gear ratio, for instance, would affect combined fuel economy by less than 1/4 mile-per-gallon. "

According to Clark, these fuel economy changes may not even be detectable during city driving. Lower axle ratios have more of an effect during long distance freeway driving. "most of the axle-ratio changes are going to have an accumulative effect over time, but it's going to be very hard to notice any difference in fuel economy where the vehicle is being driven in a mix of city and highway conditions, " says Clark.



Just thought I would inject this, since I had just read it!



Wayne

amsoilman

 

[raychem]

Does anyone know if the 3. 42 gearsets have become available for sale yet?I have been trying to buy them for over a month and no luck,any ideas?

I was finally able to get a hold of Bill Laughman @ AAM. He stated to me that they will begin shipping the ring and pinion sets starting the middle of next wk. My orders are in so hopefully they will be able to stick with that time frame.



Richard

 

[PGarrett]

Raychem is your truck a 4x4? Are the gears avalaible for the 10. 5 front axle?

 

[raychem]

Raychem is your truck a 4x4? Are the gears avalaible for the 10. 5 front axle?

Below is a post from another member with the part numbers. I do not think that they are making one right now for the 10. 5 axle, someone may in the future but that axle was only used for a short period of time. And my truck is a 4x2.

A few interested folks have asked for info about 4x4 and gears for the front diff, 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)



Front - A Diff Case and Diff Kit change is necessary for 2003 to 2006 models - use the following parts...



9. 25" Gear Set (New) - 40045288 - (used in 2007 and up)



9. 25" Diff Case (New) - 40028204 - (used in 2007 and up)



9. 25" Diff Kit (New) - 74042462 - (used in 2007 and up)

Richard

 

[raychem]

Just to let the members know, I received 3 sets of ring gears for the rearthis morning, two sets shipped today to a pair of customers that have been waiting since May, I will install my ring and pinion set next wk as I do not have the time to do it right now. I am not looking for MPG as much as a reduction in criuse rpm. I will keep you guys posted but I loved the older 3. 55 and wanted to go to a taller 3. 27 gear but never did. I have more sets on order and will be here next wk.





Note: for the 4X4 people, the front diff gear case and spider gears need to be changed... . the good thing is that the front gear set is not that expensive and the updates gear case and diff kit is also pretty inexpensive. You can contact me at 786-543-9176



Richard

 

[trqmnstr]

Raychem, any advice on switching my 3. 73 to 4. 10? Can I just swap the ring/pinion? Also, do you know the cost of the gears and approximate cost of the labor? I would appreciate any info. Thanks Clay

 

[MrBee]

I think your estimate of 2mpg is a little optimistic, but heck you may have found the fountain of youth, won't know until you try it.

I have found that every motor has a sweet spot, and that may differ from one to another even though they are supposed to be the same. I bought my truck in Sept. 06 and I just hit 60K... So I do a fair amount of traveling. I try to run the motor in the sweet spot and more than not that is where I get the best performance and mileage. On my 99 1/2 F250SD the speeds I ran were two high RPM for the sweetspot so I installed a gear vendor and lowered my RPM 15% and increased my mileage by 2 mpg. Anything that can get you 2mpg is worth it with todays pricing. Good luck and let us know!

 

[MLisle]

for those of us with trucks that are still in warranty, it sounds like we have to use the dodge dealer to install the different gears to have a warranty on them. Has anybody investigated this?????

 

[trqmnstr]

What highway speed would these 3. 42 gears put an auto at 2k rpms with stock tires? Or better yet, what rpm would I be at with stock tires and 65mph?

 

[klenger]

What highway speed would these 3. 42 gears put an auto at 2k rpms with stock tires? Or better yet, what rpm would I be at with stock tires and 65mph?

What-if to your heart's content: http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls

 

[raychem]

Raychem, any advice on switching my 3. 73 to 4. 10? Can I just swap the ring/pinion? Also, do you know the cost of the gears and approximate cost of the labor? I would appreciate any info. Thanks Clay

Hey Clay... give me a call. Are you gonna be towing heavy for the switch to 4. 10? I would no change to that ratio unless your towing max GCWR or over that.