Community Forums

NordSailOne · 08-09-2022, 10:19 AM

What are your thoughts on these two transmissions, pros and cons?
Engine is a DD16 600hp.
Towing a stacker and not towing.
Fuel economy.
Thanks.

KFed 90-10 · 08-09-2022, 11:28 AM

Ive got the 13 speed Eaton Auto Shift and really love it. Probably functions similarly to the DT12. I cant imagine having only 6 gears. I only tow a light jeep but live in the Mountain SW and frequently traverse real mountain passes 8,000 feet+, not the little hills they call mountain elsewhere. Having all those gears really shows up when going up and especially down hill. With the paddle shifter your can toggle up or down, keeping the engine in the RPM sweet spot going up or maximizing your engine braking while going down. I can maintain 65 mph going up almost any hill toggling the transmission into 12th or 11th gear at around 1600 rpms. I would say the only down side is after coming to any complete stop, it take a while to work your way through all those gears getting up to highway speed. The transmission usually starts out in 2nd and skips several gears on its way to 13. A small price to pay for all the functionality going down the road. As my engine (ISX15) has more miles on it, seems to be getting better mileage. Currently averaging above 7mpgs during the last 10,000 miles. Im 45 feet long and weigh more than 40,000 lbs.

Uncle_Mike · 08-09-2022, 12:29 PM

I agree with KFed, especially about going downhill. I think this is something we RV'ers don't think about enough, but professional truckers think about it all the time. Sure, we talk about torque and pulling up hills, but the ability to control your descent is really important, and adaptive cruise working together with the Detroit integrated powertrain of DD13/DT12/3-stage jake brake combo (DD16 in your case) really makes this easy and safe. If you're looking to pull a stacker, I wouldn't consider anything else.

I have a 38' Classic, single axle, with DD13/DT12 and I just tow a car. It climbs every hill east of the Mississippi at 60+ mph in 12th gear. I don't drive fast, usually about 68 mph (1600 rpm) just because I'm comfortable there, and routinely get 9.3 to 9.5 mpg on the highway and 8 in more urban driving. I've touched 10 mpg on dead-flat interstates like I-95 in GA/FL or I-70 in KS. And I'm happy with that fuel economy. But what I really like is the very fine-grained control of the 3-stage jake brake and 12-speeds of DT12 to control my descents.

Also, when performance mode is selected, I think the automatic skip-shifting of the DT12 is pretty peppy. I'm always up to proper merge speeds on highway on-ramps. The DD13 torque curve is so broad, it just keeps pulling, and the DT12 makes sure it always has the right gear.

NordSailOne · 08-09-2022, 02:08 PM

Thank you KFed and Uncle Mike, your observations are along the lines of what I was expecting.
I am not pulling a stacker initially, I may down the road, so I want to spec this coach as if it is my last one.
It has been suggested to me to go with the Allison as it is "smoother" and will easily handle the load of the coach.
I really think having 12 gears, instead of 6, will be beneficial with fuel economy and with going up and down hills, even it it does shift a little "rough".
I know I am ordering a truck and I am not expecting it to be smooth and plush like Prevost.

wolfe10 · 08-09-2022, 02:33 PM

OK, basic question:

Are you an "involved" driver-- prefer a manual transmission on your car? Absolutely, go with the DT12.

Are you a "put it in gear and go" driver? Absolutely go with the Allison AT.

NordSailOne · 08-09-2022, 05:38 PM

I am an involved driver, always have been.

az99 · 08-10-2022, 04:50 AM

I have a 38' Classic with the DD13 and Allison. I wish it had the DT12. It really lugs down low. Not enough gears. Allison should have at least an 8 speed. 10 would be better.

Uncle_Mike · 08-10-2022, 08:15 AM

I think in the interest of fairness (to Allison) I should add another comment:

Where the DT12 is inferior - and this is just based on my own experience - is in low-speed maneuvering, backing, parking, etc. When you need to go really slow, nothing beats an automatic. The torque converter in a slushbox allows you to feather the throttle gradually, for as long as you want, without worrying about clutch wear.

The DT12 (and I believe the AMT in a Volvo is the same way), has a "creep mode" which feathers the clutch automatically, but even then you still need a very light, gradual touch on the throttle to prevent herky-jerkiness. This is exactly the same kind of feel when you teach your kid to drive a manual for the first time (and second and third), or if you mismatch rpms during low speed shifts. If your RV storage is a tight garage, or your driveway requires a lot of backing and filling to get in or out, then this may be important to you. It's never been a factor in campgrounds. But for every minute I spend in very low speed maneuvering, I probably spend 8 hrs driving at highway speeds, so I still much prefer the DT12 overall.

Since I mentioned rpm matching, I should also say that the DT12 is an amazingly smooth-shifting transmission. Its ability to match rpms when shifting at any speeds, accelerating or decelerating, under any loads, uphill or downhill, impresses me every time. I can't shift my car any more smoothly than the DT12 shifts something weighing 10 times as much. Compared to the DT12, the automatic is a little faster on the shift just due to the inherent design of the clutch, but I really don't think it's any smoother, at least not in my ability to notice.

Uncle_Mike · 08-10-2022, 08:30 AM

NordSailOne - about your stacker intentions:
Resale is totally not important to me - they're going to have to bury me in my Classic. But if it's important to you, note that a large segment of the Classic market is made up of racers, and racers pull stackers. While you may not race, someone in the market for a used 45' Classic may very well. And those who tow stackers want the DD16/DT12.

This did not influence my decision because I wanted a shorter single-axle rig, which cuts the towing capacity considerably and will probably make mine even harder to resell (Renegade called it a "unicorn" and not as a compliment). But I don't care.

Uncle_Mike · 08-10-2022, 08:40 AM

Quote:

Originally Posted by az99

I have a 38' Classic with the DD13 and Allison. I wish it had the DT12. It really lugs down low. Not enough gears. Allison should have at least an 8 speed. 10 would be better.

I think if Allison offered a heavy duty 10 speed, I would have chosen it, especially it the gearing was biased high. The DT12 has 12 gears but those ratios are biased low, and as an RV driver I rarely use them all. In fact, it usually starts in 2nd and is into 3rd by 5mph. I'd gladly trade one for another overdrive gear. The DD13 is so torquey at low rpms, it's debatable whether we really need all those low gears in a 40K lb vehicle (which is only half the weight the chassis was designed for). Now I'm just picking nits....

NordSailOne · 08-11-2022, 10:05 AM

Thank you all for your feedback, it has been extremely helpful.
We have locked in the DT12 in our order.
Interesting to note that the price for the DT12 is $8,260 less than the Allison!

fountainhead · 08-12-2022, 09:43 AM

We just got our new rig with the DD16/DT12 recently, and my initial thoughts (compared to the Allison 6 speed in our old rig) are below...this is mostly an echo/agreement with the comments above:

The Allison definitely shifts more smoothly...much more akin to a transmission you'd be used to in your every day vehicle. As mentioned the DT12 does a really good job of RPM matching and shifting very quickly, but I was shocked at how different it feels compared to the Allison. After several thousand miles I'd say I'm/we're still getting used to it. As someone that is prone to car sickness, I tend to be overly conscientious of the passenger experience. From a dead stop with the Allison, I could put the accelerator on the floor, and it would get up to cruising speed relatively smoothly. With the DT12 I find myself feathering the throttle until it skip shifts up to 10th gear. As such, raw acceleration off the line tends to be very much reduced. Not a big deal at all...just pointing out a difference.

As mentioned, the 'creep' function, while nice, still is not perfect for tight spaces, backing into an enclosed shop, hooking up a trailer, etc. I was warned that the DT12 liked to 'lurch' a little in these situations, and, after having it for a while, I'd say that's a pretty accurate description. I find the 'creep' function much better for bumper-to-bumper gridlock traffic situations. While in the situations mentioned above, I find myself driving with two feet and using the brake pedal to mitigate the 'lurching'. Again, certainly not a deal breaker, just something to keep in mind and be aware of when you get your new rig.

I agree that all the lower gears are likely not overly advantageous for an RV application. With the Allison, I used to joke that the transmission's 'logic' must be based on a priority to get to 6th gear as quickly as possible. The same seems to be true with regards to the DT12 and 11th gear (I'd say 12th gear, but around 50mph or so, sometimes I actually have to coax it into 12th). In general, it will be in 9th or 10th gear (quickly on it's way to 11th) before you know it. One difference, and this likely has more to do with the engine than the transmission, is that most of the time when the Allison shifted into 4th, it felt like the rig kind of fell on it's face. The new rig does not feel like that at all when it hits 10th. In fact, it's just the opposite...it seems to really start pulling strongly and smoothly once it gets there. That's really nice!

At cruising speeds on the highway, I've yet to see it downshift from 12th to maintain speed on hills...ever. With the cruise set, I have seen it downshift to 10th and 11th while going downhill in order to get the most out of the engine brake, but the torque curve on these engines is so long and flat it just pulls across the entire band.

Personally, I don't think one could go wrong with either choice. I'm still getting used to the DT12, but I'm pretty sure after a few more months I will never want to go back to the 6 speed.

Enjoy the new rig!
JT

NordSailOne · 08-12-2022, 02:12 PM

JT, this is a great evaluation from someone relatively new to the DT12 coming from an Allison. It seems like it is not perfect for our use in a RV, more suited to HD trucking, but I think it will be better than the Allison going up and down hills and mountains.
Have you seen better fuel mileage using the DT12 over the Allison?
Thanks, John.

fountainhead · 08-12-2022, 02:59 PM

A few things:

1) I wouldn't say either are 'perfect' for the RV application. Both have their pros and cons, and, ultimately, it's probably a subjective decision by the owner. As I mentioned above, I'm still getting used to the DT12, but I'm guessing that before the year is over I'll be very happy I went this option. Overall, to me, it seems like a much stouter transmission.

2) My previous rig was an M2 based chassis, so a comparison of the rigs/mileage/etc is not apples to apples. I should also point out that my analysis/comparison is making an assumption that the 4000 series Allisons behave just like the 3000 series in the M2's.

3) Our new rig is actually a Showhauler (don't tell anyone as I'm a lurker here...I guess the Showhauler owners group is not large enough to justify a dedicated channel on this forum

). She's 45', and she's HEAVY...like a bit under 46k lbs HEAVY (full of fuel, fresh water, two adults, and all our stuff). That said, the DD16/DT12 combo is amazing in this rig. Flat towing my Ram 2500 I easily get 8.5MPG without even trying or worrying about speed etc. With a little extra attention 9MPG is very attainable. On one tank while heading south out of ABQ and following my buddy who was having coach problems and never exceeding 65, we got 10 and some change. Again, it's not apples to apples, but my M2 based rig never got anywhere close to that..I was lucky to break into the 7's.

4) This reminds me of one other thing - the eCoast feature of the DT12. With this enabled, the rig shifts the transmission into neutral when coasting. While cruise control (especially the adaptive cruise) works very nicely in these rigs, it doesn't take full advantage of the eCoast capabilities as it tries to keep the truck speed within a pretty tight window and leaves the trans in gear so it can utilize the engine brake if necessary. When really trying to see what MPG's I can get out of it, I find myself disabling the cruise as I crest a large hill and letting it eCoast for as long is it possibly will...you'll be amazed at how far it'll actually coast before approaching anything near an unsafe highway speed.

Apologies if it sounded like I was unhappy with the DT12 or having regrets...totally not the case. Just trying to be as transparent as possible as I scoured the Internet looking for this sort of info prior to buying the Showhauler.

JT