which axle ratio - towing rpm?

[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? Thanks for your experienced opinions!

 

[Barry]

Opinions are like belly buttons... ... ... ..... most people have one.



Spec the truck to do the job you bought it for. If you did not have the monster sized toy hauler, you would not have the 3500 Dually. Therefore, spec the truck to do the job you need it to do.



From launch all the way through the gears, an automatic Dodge used for towing big heavy trailers needs to have 4:10 gears. With a tall 0. 69 final drive ratio in OD, you just need the 4:10s. Also with the few hundred more RPMs the 4:10s will give, you'll get extra PSI in the trannie.



The 48RE is the weakest link in the DC CTD driveline, handle with care.



mi dos centavos... ... ... .

 

[HBarlow]

I think you are over thinking the issue. Buy a 4. 10 geared truck and forget about a Gear Vendors aftermarket device. I doubt the GV is strong enough to handle the torque and loads you are planning and would be a useless additional expense anyway.

A 48RE w/4. 10 gears will do the job, for awhile, then it will need to be rebuilt and beefed up.

The trailer you planned will exceed your truck's GVWR (megacabs have a 9900# GVWR) by a substantial amount and may get you in trouble with KA DOT enforcement if they ever inspect your rig. I would recommend a regular long bed dually.

 

[RVTRKN]

Opinions are like belly buttons... ... ... ..... most people have one.

:-laf:-laf:-laf



I would stay away from the stretched MC, with the weight your going to have on that rear axle, if not over GVWR, you will be at it for sure and I wouldn't trust an aftermarket fabrication of the frame for that weight. Also you might as well beef up the 48re before you do anything, and to answer your question about the gearing with a 4 spd auto, 4:10 without exception.



I would look at a C&C for your needs maybe go with a 4500 to be on the safe side. Order it the way you want and be done with it, after spending tons of money trying to stay away from a 6. 7 would not be a good decision on your part. The C&C's are trouble free, the 6. 7 in that truck with the Aison (6spd not being an option) would be perfect for your solution and you can order them fully loaded as well. Pick your gearing to match the job.

 

[HBarlow]

I was in a hurry earlier and didn't take time to answer all the OP's questions or issues.

A 37' to 40' fifth wheel toy hauler should be towed with a MDT. It is too much weight and mass for a dually pickup but I know the OP is not going to consider that advice until he has towed the loaded rig with a dually a few times and learned that lesson the hard way.

My C&C truck with Aisin six speed automatic and 4. 10 gears is very comfortable at 62 mph in fifth gear (tow haul mode, sixth OD locked out) and running around 2100-2200 rpm when I am towing my fifth wheel which weighs 14,000 lbs. My GCW is around 25,000 lbs. A 37' to 40' toyhauler will be far heavier than my rig.

Fuel economy is not an issue when towing a heavy fifth wheel with a massive frontal area pushing wind like a highway billboard. What does matter is safe towing speed and proper gearing to have the engine at it's most efficient rpm. Cummins says the most efficient engine operating speed is 2150 rpm. That is in the middle of the torque band and well up the horsepower curve. At a combined weight of around 25k# my truck will maintain gear and speed on moderate grades without strain. With the heavy toyhauler you mentioned you will be in direct drive 3rd or down in second on most grades with a ISB5. 9/48RE combination. Additional gears will be of no use.

Contrary to old wive's tales and popular belief, fuel economy is only a tiny fraction better with 3. 73 gears over 4. 10 gears when unloaded at highway speed. If anything, the 3. 73 gears are probably less efficient when towing because more right pedal input and more frequent downshifts and high rpm pulling on grades is required. 3. 42 gears are just a poor choice for heavy towing.

A truck should be spec'd for the load it will carry. Worry about fuel economy on a daily driver. If you compromise on the truck it will do a lousy job towing a heavy trailer and may also be unsafe as a tow vehicle.

Exhaust Gas Temperature is never a concern on a factory tuned unmolested engine. The HPCR engines '4. 5 and later were designed and tested to run continuously at 1400* or higher when operating at full power under full rated load. You won't have the luxury of backing out of the go pedal on grades to keep EGTs low with the load you are contemplating.

 

[ILIANBG]

If I were to get another truck it will be a newer 6 sp. auto. How much newer- as much as I can afford. Ideally 2010. I love the new body style. And definitely 4. 10 axle. I hear great things about the newer trucks with 6 sp. autos for towing and heavy loads.
Good luck.
Ilian

 

[JohnBoy5]

i will agree with the folks that have spoke before. thats a LOT of toy hauler and 4:10's is a must on a stock automatic. How ever, i have 3:54 and pull 20k to 25k loaded deck over gooseneck up some pretty stiff hills. I do have a very modified trans and truck though. I have never seen over 1200°F on my egt mounted before the turbo.



I also do NOT have the wind drag that you will have with your trailer... .



Hope this help

 

[Road Dog]

The back seat of my friend's QuadCab is just fine on extended trips. Unless you family has been supersized I'd skip the MegaCab.