Which rear end to get 3.42 or 3.73 ?

[LocoPilot750]

I've been driving my 2WD '97 2500 for almost 400k miles. It has a 3. 54 rear end, and when stock, had 245/75-16 tires. It's mostly a highway truck, so went with the higher gears at the time. Right after I bought it new, I changed the governor springs, put in a TST kit and bigger injectors, and it puts down 300+hp/700lb torque on a floor dyno. I later upgraded to larger diameter 265/75 tires, and that had the effect of gearing the truck up even more, but it had all the power it needed to handle that at all speeds. (It only weights 5900 lbs, so has a high HP to weight ratio). In locked up OD, the engine turns 2000 rpm at a true 75 mph. This is about where I like it to run on the highway. NOW... . I will be ordering or buying a '14 Laramie 4X4 this fall, and I'm trying to figure out which rear end to get, the 3. 42, or the 3. 73. With the stock 18" tires, in lockup OD or 6th, what will the tach be reading at 75 mph with either rear end? I'm thinking the HP to weight ratio might be pretty close to where my old truck is now, so I'm looking for that same RPM/MPH feel I have now. I pull a little, no real heavy stuff, the truck is mostly a long distance highway traveler. I've been out of the TDR for about ten years, but I know someone here has an answer for me.

 

[crabjoe]

If you're getting another SRW truck, you won't have an option... 3. 42s are it for SRW Cummins Rams...

I just pick up a 14My 3500 w/Aisin. I'm used to having 4. 10s on a 2004 2500 and I'm wishing I had the 4. 10s on my '14 truck... The trucks feels a bit sluggish with the 3. 42s, but it might be the lag, more so than the actual power. Off the line is ok, but passing/merging is a royal pain. The motors just not that responsive... I'm hoping that the motor live-ins up a bit once I get some miles on her... I just picked up the truck Saturday evening.

 

[LocoPilot750]

I'm looking at the 2013 sales brochure, and you're right, for a 2500 4X4 with automatic, it looks like I can get anything I want as long as it's a 3. 42. Kinda hoping they might change that with the '14's, but with NO SALES BROCHURE's available at any of the dealers I've visited, how's a guy gonna know? I can't just sit down with the salesman and spend $60k without really studying this stuff, and knowing exactly what I'm buying. But that's what they want me to do. It might be nice to have the lower gearing option and be able to go up a size or two with tires. With 3. 42's, bigger tires might not be a good idea. I was hoping at 370 HP the engine would already be a little more lively than what I have now, or at least as good as. The old 5. 9 I drive right now, makes me smile every single time I drive it. It's very throttle sensitive, especially in lock up OD. Just a slight touch of the foot, and amazing things happen. I'm hoping a new one feels the same way.

 

[Joseph Donnelly]

I expect with the "carbon emissions" game from the feds, 3. 42s will stay. The good news is that with the 385 HP/Aisin, it works well. RPM is about 1670 or so at 75 mph but power is good. There is a bit of lag on takeoff, partially the gearing but partially the fueling before boost begins to meet the low emissions mandates. It isn't bad. If you want more rpm, just click down to 5th on the highway. You will get used to it going in 6th. Remember you now have 408 cubic inches, not 359.

 

[Hoefler]

My 2010, with the 4. 10 ratio will turn just under 2000 rpm at 70 MPH. Everything stock.

 

[RustyJC]

2011 3500 DRW with 4. 10s - 2015 RPM @ 75 MPH



Rusty

 

[LocoPilot750]

If you get 75 mph with 4. 10's, then there must be another factor besides ratio. On my old 97, as far as I know, in lock up OD, the crank shaft becomes welded to the drive shaft, everything turns at the same speed. I think with the later six speeds, in top gear, the drive shaft will actually turn faster than the crank?

 

[RustyJC]

Right. 6th gear on the 68RFE and Aisin is 0. 63:1 ratio, so if the crank is spinning 2015 RPM @ 75 MPH, the driveshaft is turning 2015/0. 63 = 3198 RPM in 6th gear with the torque converter locked up. The double overdrive transmissions are total game-changers as far as axle ratios are concerned. No longer does cruising at 70 MPH with 4. 10s mean 2350 RPM like it did in my 2002 NV5600 dually.



Rusty

 

[cerberusiam]

On my old 97, as far as I know, in lock up OD, the crank shaft becomes welded to the drive shaft, everything turns at the same speed.

Nope, only in direct. With any transmission in OD the drive shaft is spinning faster than the crank, varies by transmission. Been that way since 89 with the manuals and 91 with the autos.

The difference in the autos is . 69 in all the 4 speeds, and . 62 or . 63 in the 6 speeds. Manuals have a lower ration with only 1 OD gear. 5th gear is around . 8. The 6 speeds split the old OD ratio nicely.

If you are just road driving and not towing heavy the 3. 42 should do nicely for economy.

 

[SSteilow]

Sorry to ask this question not trying to hijack the thread.

How about if you are Hauling a non-slide Truck camper up in the Mountains of Colorado, Wyoming and Montana with the 3500 Crew Cab and the 3. 42 SRW. Seems you would not have the Power to climb the mountain passes unless you had a 3. 73 or 4. 10, according to the build guide, which states for mountain driving. I guess the other options is to go with the Dually then you can get the 3. 73 or 4. 10.

Thanks for your replys,

Scott

 

[MelvinPierce]

Sorry to ask this question not trying to hijack the thread.
How about if you are Hauling a non-slide Truck camper up in the Mountains of Colorado, Wyoming and Montana with the 3500 Crew Cab and the 3. 42 SRW. Seems you would not have the Power to climb the mountain passes unless you had a 3. 73 or 4. 10, according to the build guide, which states for mountain driving. I guess the other options is to go with the Dually then you can get the 3. 73 or 4. 10.
Thanks for your replys,
Scott

With only the wt of a truck camper, I cannot believe you cannot climb any hill that the pickup will go up anyway. 1st gear is really low, plus the torque multiplication of the convertor, you can climb to the limit of your traction. If a 3. 42 will not climb it, I doubt a 3. 73 or 4. 10 will. A truck camper is pretty light compared to a possible 30,000# trailer!

 

[Joseph Donnelly]

The numbers I gave are with the new 18" tires that have a nominal 33. 1" diameter. My calculations (see Issue 81) show about 2100 at 75 mph with 17" wheels and 4. 10 in 6th with Aisin or 68 RFE.

 

[RustyJC]

My calculations (see Issue 81) show about 2100 at 75 mph with 17" wheels and 4. 10 in 6th with Aisin or 68 RFE.

I'll pull up my Excel spreadsheet when I get home and see where our assumptions might be at variance.



Rusty

 

[SSteilow]

With only the wt of a truck camper, I cannot believe you cannot climb any hill that the pickup will go up anyway. 1st gear is really low, plus the torque multiplication of the convertor, you can climb to the limit of your traction. If a 3. 42 will not climb it, I doubt a 3. 73 or 4. 10 will. A truck camper is pretty light compared to a possible 30,000# trailer!

Thank you for the reply Melvin.

Should be good to go with the 3500 4X4 SRW 3. 42 along with the Wolf Creek 850 Truck Camper.

Scott

 

[LocoPilot750]

So far, I cannot find a dealer that has the 2014 brochure, with all the grids for the options, but I do have one for the '13's. It tells me 17, 18, and 20" wheels and tires are available for the 3500's variations, and only 17's are available for the 2500's. However, I just went to the "build and price" page, and it's telling me I can have 18 or 20" now for the 2500, but not 17's. Is that correct, or is it a glich in Ram's build page?

 

[Wingate]

So far, I cannot find a dealer that has the 2014 brochure, with all the grids for the options, but I do have one for the '13's. It tells me 17, 18, and 20" wheels and tires are available for the 3500's variations, and only 17's are available for the 2500's. However, I just went to the "build and price" page, and it's telling me I can have 18 or 20" now for the 2500, but not 17's. Is that correct, or is it a glich in Ram's build page?

me either. No brochures yet. Website doesnt offer some of the options that Ram is suppose to be bringing to market either.

 

[EDankievitch]

I've got the 3. 42's in my 2013. Wasn't sure how I would like them... Have to say now that I have 4,600 miles that I LOVE them Fuel MPG's have been around 22. 4 Hand calc. on long trips at 70 Mph. Local Mpg's run around 17-18. 5 performance is good never feels like it's lugging. I think RAM got it right this time.

 

[rrl989]

Thank you for the reply Melvin.
Should be good to go with the 3500 4X4 SRW 3. 42 along with the Wolf Creek 850 Truck Camper.
Scott

With the DRW, I had a choice and went with 3. 73 due to the AF 990 TC. I'm sure the 3,42s
would have been OK, but my choice will only help with my setup.

 

[RustyJC]

Joe,



As promised, here is the data from my spreadsheet that's behind my numbers:



Tire size: LT235/80R-17

Tire revs/mile: 634. 1552

OD Ratio: 0. 62



The OD ratio is from a 2011 source and looks like it's off by 0. 005 (as is an assumption of 0. 63) since the actual ratio on the 68RFE is apparently 0. 625:1. If I use 0. 63 instead of 0. 62, I come up with 1638 engine revs/mile versus 1612. The only other difference I could see would have to be the tire revs/mile you're using.



Rusty

 

[SSteilow]

With the DRW, I had a choice and went with 3. 73 due to the AF 990 TC. I'm sure the 3,42s

would have been OK, but my choice will only help with my setup.

Thank you for the information.

The AF 990 is my second choice. If i go that route it will be with a DRW and the 3. 73.

I like to stay Flexible in my choice's, so i get the correct Truck setup with my TC.

Scott