Changing from 3.73 to 4.10 gears

4x4ord wrote:

goducks10 wrote:
I very rarely see 2nd with my 6.4, 4.10 gears towing 9300 lbs in the mtns of Oregon. Only on the steepest curvy sections. Most of the time I'm cruisin in 4th with some 5th and some 3rd mixed in. Freeway is all 5th till I hit some hilly areas.

If you have 32" tires you will be running 3650 rpm in 3rd gear at 60 mph. The 6.4 Hemi makes about 290 HP at that rpm. You should be able to tow your trailer up a 6% grade at 60 mph in 3rd gear. Anything over 6% and your in 2nd ... I would say your truck/trailer combo is a pretty good match.

You're pretty much spot on as I use 3rd for those steeper sections.

33.2" tires OEM
1.41 3rd
4.10 final
60 mph
3510 rpms

goducks10 wrote:
I very rarely see 2nd with my 6.4, 4.10 gears towing 9300 lbs in the mtns of Oregon. Only on the steepest curvy sections. Most of the time I'm cruisin in 4th with some 5th and some 3rd mixed in. Freeway is all 5th till I hit some hilly areas.

If you have 32" tires you will be running 3650 rpm in 3rd gear at 60 mph. The 6.4 Hemi makes about 290 HP at that rpm. You should be able to tow your trailer up a 6% grade at 60 mph in 3rd gear. Anything over 6% and your in 2nd ... I would say your truck/trailer combo is a pretty good match.

Dbl post

I very rarely see 2nd with my 6.4, 4.10 gears towing 9300 lbs in the mtns of Oregon. Only on the steepest curvy sections. Most of the time I'm cruisin in 4th with some 5th and some 3rd mixed in. Freeway is all 5th till I hit some hilly areas.

Thanks for all that math. Which brings up my point about these "wonderful" 6-speed transmissions we have these days: They added more overdrive gears for better mileage when empty and more gear choices for mild loads & hills but when really pulling hard with a gas engine you're back to the 2nd and 3rd gears of yesteryear. So the rear axle ratio still matters for specific situations where you're stuck between those gears. Maybe the 2-3 gap is narrower? I say bring on the 8 or 10 speeds!

To move out of the weeds and apply things to the OP's situation I think the OP needs to realize that the 6.2 liter engine makes 316 HP at 4200 rpm and 385 at 5500. If his loaded truck and trailer weigh in at 22000 lbs he should be able to pull a 6% grade at about 55 mph so long as he can keep his rpm above 4200.

55 mph means the rear axle is turning 578 rpm. If he goes up the hill in second gear with 3.73 rear end his engine will be turning 5002 rpm (2.32:1 2nd gear ratio x 3.73 x 578). At 5000 rpm he might even have enough power to accelerate to 5500 rpm (60 mph) where the engine makes peak HP.

So according to my math the 3.73 gears should allow him to put full power to the road at 60 mph on a 6% grade in 2nd gear.

If the hills are steeper than 6% he might benefit from a 4.10 rear end which would allow him to put full power to the ground at 55 mph.

Our Kenworth is a tridrive double framed truck designed for very heavy work. It has a much lower speed final drive. Top end for this truck is about 75 mph. We use this truck mainly off road. The other is a highway tractor. It is comfortable rolling down the highway empty at 90 mph but when it is left parked in the dirt with 100,000 lbs hooked to it, it has a hard time getting itself moving.

Once on the highway and up to 65 mph both trucks perform pretty much identical. I run through the gears often skipping at least one gear on every shift and often skipping two gears when unloaded. The flat HP curve of these engines coupled with 18 speed transmissions means I have no trouble keeping the engine output at 550 HP no matter the road speed.

My 440 HP Ford pickup on the other hand only makes 440 HP at 6 specific speeds ... 2800 rpm in each gear. If the Powerstroke is revved to 3200 rpm in 3rd prior to shifting to 4th the power will likely have dropped to somewhere around 390 Hp prior to shifting and after the shift, at 2400 rpm, the engine will still only be making somewhere close to 390 HP with the throttle to the floor. More gears or a flatter power curve would allow the engine to produce closer to the rated 440 HP at all highway speeds. This range was much larger even a few years ago. The huge torque of these new pickups means they do have a significantly flatter power curve than they used to.


Edit: I've noticed that when climbing hills the Powerstroke likes to hold onto gears longer. For instance when pulling up a grade with my Ford, the Torqshift would make best use of the Powerstroke's power band by dropping out of 4th gear at 2300 rpm. Instead it will hang onto 4th gear and let the engine pull down below 2000 rpm. I don't mind this at all but I do think that the shift points of the Allison/Duramax are at higher rpms, making better use of the Duramax's power and contributing to its success on the Eisenhower hill runs.

So 4x4ord, if your class 8’s and trailers were identical as possible, with those C15’s, except one has 3.70 and one has 4.30, which one pulls 140,000 lbs / 8 axles at 60-65 mph “easier” in all gears and conditions except downhill with a tail wind?

wnjj wrote:

Huntindog wrote:

4x4ord wrote:

garyp4951 wrote:
Yes your torque will be the same, but a higher number gearing will always give more torque.
I was at a 4x4 truck pull where they see who can pull the other one backwards, and a old 1980 GMC 2500 with 4:56 gears, and small tires was pulling new Fords, Rams, backwards while they were smoking their tires.lol

Like I said I can downshift, which provides a higher number gear ratio resulting in the engine speeding up. This yields the same torque on the rear axle because the Cummins ISX 15 liter engine has a flat power curve over a three gear range of the transmission.

A flat power curve makes it even more impossible.

Show your math

A flat power curve (i.e. same HP at the different engine RPM’s in each of the 3 gears) means the same wheel torque in all 3 gears since the road speed is the same. Same power equals same wheel torque applied at the same rate (road speed).

In order to have a truly flat power curve, the torque curve is decling inversely proportional to engine RPM. This means a downshift results in higher RPM (lower torque) which is then multiplied back up for the same wheel torque.

Yup,

Here is the power and torque curves for our Cat C15. You can see from the graph that the power is flat from about 1400-2100 rpm.

Oh...on the "mine can beat the poop out of yours"...

Even if everything was exactly the same from the factory/OEM...there are tolerance stackups that make them different

Then folks like me who use synthetics in everything...vs dino, there is less friction

On top of that, I break in my ICE's with MolyB powder and reapply often, which has a molecular bond to the rubbing/sliding/etc metal to metal parts...there is no metal to metal contact. Even if it goes dry, there is a MolyB to MolyB contact, which has a lower coefficient of friction than an oil/synthetic film

Plus other things done. Every little bit helps...