I wish we could go back to the days of ordering the ratios we want.

If we could, there would be a wide track 350 or a 450 with 4.10's in my driveway.

 

The mountains was the only short comings I had with the 6.7 this last summer.


Did pretty good over Monarch pass but over the Eisenhower tunnel and the summit of Aspen I was on the floor maxed out at 50-55 for several miles.


But that is due to the turbo. I need more air. They fixed that for the '15 model year. Pickuptrucks.com has tested it and it rolls some coal now.

 

Somebody like Tom could probably lay out the final drive ratios between the mentioned ratio/tranny combos.

4.10/5 speed
4.30/4 speed
3.73/6 speed

That what been talked about?

I would guess that with 18" wheels, 4.30 in the current 6 speed transmission would be an absolute monster but mainly for urban work trucks.

If I were traveling over the road with a heavy camper I would stick with 3.73 in a new truck. It's comfortable, may be sluggish on hills, will be nice cruising though. 2000 rpm at 70mph is prob what you will see and I'd say that's just over what would be optimal for economy and noise.

 

Had to chuckle a bit about that one Scott. Lots of passenger cars have a hard time going over 50-55 MPH up those grades. I overheated a perfectly functioning Kenworth up to the Eisenhower tunnel back in '09 at 79,500 lbs. That was at 25 MPH with the pedal on the floor. 50-55 pulling that trailer of yours sounds pretty good to me!

Hi Joel!

I'd be happy to as I've done in the past, but I think the general principle is easier to remember without all the math. The more gears you have in the transmission, the less rear end gearing matters.

From a practical standpoint all that matters with the new 6-speed is your first and sixth gear ratios and how they work with your rear end ratio. That's because there are enough ratios in between for the transmission to put the engine right where it needs to be at just about any speed. Of course the upper limit to that is 6th gear, so if you pick too high of a rear end gear you're turning higher than optimal RPMs on the highway, and without a seventh gear the transmission can't adjust for it.

I'd suggest 3.73s with the 6.7L engine. If you want the feeling of steeper gears just lock out 6th gear. In 5th gear you have the same effective gearing as 6th gear does with 4.56es!

 

I've had this discussion before. No one has changed my mind yet. So, I'll try it again. I do not believe Transmission gearing can replace the rear axle power. They are two different things. There is some level of work that a 4.30 will over come, but a 3.73 will not, no matter how the tranny is geared. It is very doubtful we will, in our trailer towing ever encounter such an obstacle, but that does not alter the fact that it exsist. If that were not so we wouldn't be having this discussion, all rear axles would be 3.55 or higher and low end power would be determined by the transmission gearing.

 

I think that it does not matter where the gear ratio multiplication is done - either the rear end or the transmission. It all ends up with the number of revolutions in the crankshaft to the number of revolutions of the rear wheels.. With today's 6 speed transmissions, the ratio of the rear end is far less relevant than it was with a 4 speed and no OD etc.. The gear multiplication to get the load rolling is they key here. Get the engine to it's peak torque as quickly as possible and maintain it there. For these engines, that is about 1500 RPM if I remember correctly. Running it at 2000 RPM is counter productive and wasting fuel IMHO unless you need those RPMs to carry the speed you are after...

Me - I tow at 65 MPH..

 

^^^^^^^^^^^This is very true. There will be less strain on the whole drive train with deeper gears; Transmission, TC (if 4WD), drive shafts, u-joints, etc.
I am pulling with 3.73's and the 6.2 gasser; not ideal, but it definitely gets the job done, and gets me around 8 MPG while pulling either of my loads. The 6.2 loves RPM, so I was "thinking" the 6.7 PSD would like a lot less RPM. But then, maybe the whole shifting strategy of the 6R140 in the 6.7 is programmed differently than with the 6.2, so that it wouldn't be spinning nearly as many RPM. This is one of the bases for my inquiry.

 

That's not true for a variety of reasons. A differential is nothing but a gearset; it doesn't make power.

Here's an example: This was my last road tractor before I entered active duty again. Rated for 80,000 lbs GVWR, it had 3.25 rear gears.



It could do this because it had a 16:1 first gear and a 13-speed transmission. If you could connect that transmission to the engine in your pickup truck it would make a 2.73:1 rear axle do things a 4.30:1 wouldn't do with the 6R140. It's just gearing.

Rear axles have different gearing because it's cheap to do so. Changing the ratios in an automatic transmission would require a complete redesign of the transmission. It would be a hundred times more expensive to regear a transmission than it is an axle. So when truck manufacturers adjust gearing for an application rear axle gearing is the logical choice. That doesn't mean it's the only way.

That's true, but I'd question the practical value of this. Transmissions rarely fail from being over torqued; typically it's a hydraulic failure of some flavor that kills them. U-joints fail from lack of lubrication in all but the most extreme cases. Would any of these components really care if they saw ~10% less torque in a given application?

 

Lol, where's that popcorn eating thing?

I can understand individual preferences and certainly don't judge y'all's decisions, but, I will always choose a 3.55 with 20" wheels or a 3.31 with 18", if it were available on a dually. So I'd always offer the 3.73 advice. For leisure travel always go with economy.

And yes, peak torque is around 1600 I believe. Peak horsepower is redline I think. The two are very happy together between 2,000-2,400 with my 3.55 rear end. I cruise at 72mph, approx. 1700 rpm I think. Weight never changes my fuel economy figures, wind resistance always does. If I'm towing 15k or 6k, it's all the same to the drivetrain, prevailing winds and frontal area are where it's at.

 

I seem to turn around 1800 rpms at 65 mph when towing.

 

OK. Lets try a different angle on my original question. With my 6.2 gas and the 6R140 tranny and 3.73 gears, when I get into a good pull it will drop into 3rd and sing along at 4000 to 4500 RPM to maintain road speed; which the 6.2 is designed to sing. Now, will the 6.7 just chug up the same pull without downshifting as far, therefore requiring a lot fewer RPM. I.E. Is the shift strategy programmed altogether differently?????

 

Absolutely.

You'd probably climb that same hill around 2,000-2,200 RPM depending on speed. The 6.7L makes max power at 2,800 RPM, and the transmission won't let it get much over 3,000 RPMs at WOT. Redline is around 3,800 RPMs if I remember right.

 

OKAY!!!!!!This helps a lot. So when I get around to ordering/finding what I am looking for, it will probably have the 3.73's in it then.
Thanks a bunch, and have a GREAT day!!!

 

I have always wondered why most people want to race out and put the lowest gears they can find, and then turn around and buy the truck with the largest factory tire size available. Then brag about how much they can pull based only on gear ratio.

I have 3.31s but with the smaller 17 factory tires and pull similar loads with little issue.

The gear ratio discussion is much ado about nothing if you aren't including tire size in the mix.

 

If you watch video from our last truck meet you can see Senix's truck at work towing 15k up a 15% grade. Granted the grade is not long but it was from a stop. He was still able to spin the tires with 3.73s.

If I was ordering a DRW I would get 3.73s but of course I would also get 4x4. In end order what you think will better match your needs for how you plan to use your truck.