What axle ratio/rpms for towing/egt control?

[TFlewitt]

Our family is going to start RVing with a 5th wheel toyhauler (37-40') and are moving from a '99 2500 LB to looking for an '06/'07 3500 DRW 4X4 Mega Cab (stretched to LB) with a 5. 9/48RE.



I considered the 6. 7/68RFE (for the gear splits of the 6 speed), but decided to stay with the 5. 9 and are considering a Gear Vendors to split gears on the 48RE (manual trans not an option for us).



I'd like to hear from some of you concerning towing speeds/rpms and rear axle ratios. I have an Excell spreadsheet showing ratios/mph/rpms.



What speed do you tow at? What rpms do you find work well (at 55-65 mph) for controlling EGTs with a big toyhauler?



We've found a MC LB with a 3. 73. At 55 mph in 3rd, the rpms are 2150, in overdrive they are 1500 (below torque peak). With a 4. 10 at 55, they are 2400/3rd, 1650/4th.



Without the Gear Vendors, I agree the 4. 10 is better for towing, but not as economical unloaded (where it will see a lot of use).



What is your opinion though with the Gear Vendors? A 3. 73 in 3rd over would be 1700@55 whereas the 4. 10 would be 1850@55mph. Would a 3. 73 be workable with a Gear Vendors and big toyhauler? I'm concerned with egts, level towing power, and gear hunting by the 48RE. Thanks for your experienced opinions!

 

[AH64ID]

Even with a gear vendors I would run 4. 10's for your use.

 

[TFlewitt]

Even with a gear vendors I would run 4. 10's for your use.

Could you elaborate? Do you tow medium to heavy with 4. 10s? Have you towed with the 3. 73s? What is your egt/rpms/weight/speed? Thanks!

 

[JJPage]

I agree with the 4:10's, especially if you are going to use a Gear Vendors.



I tow my 30ft 5vr with 3:73's, 3rd gear is to low for the hiway, and in OD I find myself slowing down for corners, even uphill on the interstate. OD my truck wants to pull at 70/75+mph, that's ok across eastern Montana, but I'm not comfortable in more congested areas. I used to drive semi otr(15yrs), and a 37/40' trailer behind a 3500 DRW is really pushing it IMO.



I believe 4:10's would be your best bet, all things considered.

 

[NIsaacs]

With the Gear Vendors, go with 4. 10's. You still have a good empty cruise gear with the double overdrive. Without it, go with 3. 73, 4. 10's are too slow when you are empty.



Nick

 

[macdaddy]

Whew... .



You will get allot of feed back on this one. It sounds like youve done some good research already.



I will say this though, since your choosing a slush box (auto) your options most certainly are limited there by needing a under/over drive (which will help). The trailer your considering will require lower gearing. Your the only one that knows how much your truck will be used for towing.



When choosing a transmission gear ratio just chose a set up where your in 3rd/direct drive so the cummins is between 1900 to 2100 RPM's. Going all out with a Gear Vendors/US Gear under/over drive will allow you a nice setup, however unreliable (in my opinion).



I would go with the 4:10's and use the 3rd over for towing the monster your going to be hauling. I think youll find the RPM's right in the ball park for max torque and fuel economy.



Mac

 

[RustyJC]

We ordered our 2002 3500 dually specifically configured to get the maximum available GCWR, 21500 lbs that particular model year. This required the Cummins HO, NV5600 6-speed manual and 4. 10 gears. With this setup, 60 MPH in 6th gear was about 1950 RPM, 65 MPH was 2150 RPM and 70 MPH was 2350 RPM. I understand that the HPCRs are somewhat of a different beast, but that tells you where Dodge/Cummins wanted engine RPM when towing heavy. Typically, pulling our 16000 lb GVWR 5th wheel, you could get it into 6th gear, set the cruise and forget about having to downshift.



The new truck (see signature) was also ordered to get the maximum available GCWR (24500 lbs) but is turning only 1612 RPM @ 60 MPH in 6th with 4. 10s, which seems really low to me because of the double overdrive and really high 6th gear (0. 61:1) of the 68RFE, but with the 3. 73s it works out to a calculated 1488 RPM at 60 MPH which is even farther below the sweet spot. I guess we'll see how this configuration works out when we hitch up the 5th wheel and head out in a few weeks.



Rusty

 

[HBarlow]

I answered your question in your duplicate post in the RV/Towing/Hauling forum.

Rusty, I find my '08 C&C with Aisin six speed and 4. 10 gears most comfortable for myself and the power/drivetrain using tow/haul mode with sixth gear/2nd OD locked out. Sixth gear is great for high speed unloaded highway cruise but I usually don't want to tow a heavy fifth wheel at those speeds.

 

[RustyJC]

Harvey,



I agree - about 65 MPH is my typical maximum cruising speed. If tow/haul locks out 6th (I haven't tried it yet), then everything ought to be sweet! :-laf



Rusty

 

[AH64ID]

What I would elaborate on is what others have said... 4. 10's, truck in 3rd with GV engaged will be a great towing setup.

 

[JMarketos]

I have an 05 QC 4x4 (stock tires) with 48RE and 3. 73 and GV unit installed.

My experience seems consistent with the thoughts above here:



[1] Towing in 3rd gear (with 48RE) really isn't an option for highway speeds because engine RPM would be far too high (ie near redline). Part of the difficulty with gear ratio selection in this trans is that there's a large % difference in ratio between 3rd and 4th gears.

[2] 4th gear is too stiff for towing anything heavy. (around 1750 RPM at highway speeds) The truck will do it, but EGTs easily get uncomfortable. Also if you use cruise control, you can get into situations going up long grades, where the CC adds boost and adds boost and adds boost... then depending on grade if it detects the rig is continuing to slow down (meanwhile, EGT's are glowing... . ) it will downshift to 3rd which (see above) results in the tach redlining. This is accompanied by a big boost spike, which can pop the plastic side tanks on the intercoolers that were used on some 05 models. (not to mention the driveline shock... )



I did the calculations and purchased a GV overdrive unit. I made a separate post many months ago, where I think I detailed more of that. With this setup, run the 48RE in 3rd (which is 1:1 straight through), and engage the GV overdrive. This ends up with the engine at 2100 -2200 at highway speeds, which seems much more suited to the engine's calibration and also helps EGTs.



I tow a very light trailer (20ft car hauler) compared to most here- about 6K-7K fully loaded. In the stock configuration (no GV, tow in 4th 1750RPM), I got around 11-12 MPG highway. With the GV unit installed, same highway speed or maybe even a little faster, running at 2100-2200 RPM but much lower boost, I got 14-15MPG.



Keep in mind with a GV unit, you cannot use engine braking when the GV unit is in overdrive mode.



I also installed a Banks package including intercooler at a later date. Specifics are tough to tie down for EGTs, but I can tell you that just the GV unit alone helped to reduce EGTs via the increased engine RPM and lower boost needed for a given speed. I can also state for sure that the increased intercooler made a big (further) difference in EGTs.



A 37' - 40' trailer strikes me as on the large side to pull with a CTD, but I guess it's a matter of what is the gross weight. Anticipating that you'll be pulling at least 2x what I do, if not 3x or more, I'd be inclined to echo the recommendation of others and consider seeking a truck with 4:10s AND the GV unit for your heavy towing needs. When unloaded, you'll be able to use 4th gear and the GV.



Also I'd recommend to consider an upgraded intercooler, as they do help with reduced EGTs.



Best of luck in your pursuits.

 

[ENafziger]

Towing heavy, my truck likes 2050 - 2300 rpm best. It never labors going up hills, and doesn't get hot either.

--Eric

 

[Prairie Dog]

3:73 guy here ~ it works great for me but your looking at nearly twice the weight I pull. 4:10 will keep you in the power a lot better and MPG wise, isn't that much of a difference from the higher gearing. Another vote for 4:10's!

 

[HBarlow]

Harvey,

I agree - about 65 MPH is my typical maximum cruising speed. If tow/haul locks out 6th (I haven't tried it yet), then everything ought to be sweet! :-laf

Rusty

Rusty,

My Aisin is programmed a little differently than yours. My truck doesn't have the handy toggle shifter on the column to change gears at will. You will probably decide to select Tow/Haul mode and toggle down to fifth which I understand will make fifth your top gear locking out sixth.

Fifth gear/first od is a great gear for towing with my Aisin and I think your 68RFE will be similar. Fifth gear ratio falls between fourth direct and the extreme overdrive ratio of sixth.