Rear End Ratio vs. MDS Operation

[Q68]

Hi all,

I've been lurking here for a while and considering buying a new Ram 4x4 CC with the Hemi. Sorry if this is a bit long-winded, but I'm trying to figure out which gear ratio I want, and I'm wondering about its effect on the MDS system operation.

I test drove a CC short bed with the 3.21 in it, and I was underwhelmed by the performance - very lazy throttle response unless I really put my foot in it, and the trans needed to drop at least 2 gears at highway speed to get decent acceleration. This got me to thinking about the MDS system - I would think that with the 3.55 or 3.92, the shorter final drive ratio might allow the MDS to operate over a wider speed/incline range vs. 3.21, unless there is a hard RPM cutoff above which the engine will always run on 8 cylinders.

So, can some owners with each of the available gears chime in and let me know about how often, and under what conditions, you see the MDS operating? Is there a set maximum engine or vehicle speed above which it won't work?

Thanks in advance.

 

[noupf]

Hi all,

I've been lurking here for a while and considering buying a new Ram 4x4 CC with the Hemi. Sorry if this is a bit long-winded, but I'm trying to figure out which gear ratio I want, and I'm wondering about its effect on the MDS system operation.

I test drove a CC short bed with the 3.21 in it, and I was underwhelmed by the performance - very lazy throttle response unless I really put my foot in it, and the trans needed to drop at least 2 gears at highway speed to get decent acceleration. This got me to thinking about the MDS system - I would think that with the 3.55 or 3.92, the shorter final drive ratio might allow the MDS to operate over a wider speed/incline range vs. 3.21, unless there is a hard RPM cutoff above which the engine will always run on 8 cylinders.

So, can some owners with each of the available gears chime in and let me know about how often, and under what conditions, you see the MDS operating? Is there a set maximum engine or vehicle speed above which it won't work?

Thanks in advance.

i too am planning to buy a new sport and i plan to do the 3.92. My dealer locally has two nicely loaded ones and i saw they had the 3.21's and i said NOPE. Dealer tried telling me its not a big difference, i said, yes, yes it is. Just from a quick test drive i can tell as well.

My 2010 silverado ext cab has a 6spd with 3.08's.....wanna talk about a dog of a truck.......I loved the truck, hate the performance. i just never thought about it when i bought it new. I've been mad at myself since.

My buddy has a 14' sport with 3.92's and its great. He and I do a lot ( maybe 60%+ ) low speed, start stop driving ( and a little bit of trailering..... 3500lbs loaded triailer ) and the 3.92 really helps out with that when we use my his Ram. My silverado is just a slug unless you really give it gas. Plus, my eco mode (mds ) acts very funny in my silverado with those low 3.08's. He sid he cant tell when his kicks in, but i dont have speed or rpm info for you. My truck almsot has a slight shivver running through it as im trying to cruise at 65mph and as the 4 cycl mode is trying to do all the work ( and this is whithout the trailer ).

Others here will say the 3.21 is ok and i bet that since it matted to the 8spd tranny, its not nearly as bad as my silverado, but i already know that it would be underwhelming to me too.

 

[nomadsmc]

I had a 2012 Bighorn QC with the Hemi and 3.21s and a 6 speed. Did a lot of downshifting on the slightest grades when at highway speeds (65+). Fuel mileage was pretty good though, in the summer 22 mpg, winter 16.5-17.5. My 2014 Ram Outdoorsman QC has the 3.92s and 8 speed and accelerates much better and holds speed on grades a little better without downshifting. So far fuel mileage is 21 in 40-50 degree weather and 18 when sub-zero. Can definitely tell the difference between the two.

 

[Copperhead7310]

Just push the Tow Haul button on the dash when you test drive them both. It will disable the MDS and you can get a true feel of the trucks, one vs. the other.

It really just depends on what you want, fuel economy or a little more in the seat of your pants. The 8 speed 3.92 is quite impressive in my opinion. I have never driven a 3.21 so I dont have a fair measure though.

Even with the 3.92 I'm avg 23mpg on the freeway.

 

[7777xm]

MDS is something that you may get a feel for or hear, but there is no visual indication whether it is on or off.

I've driven a couple different rear end ratios in really never noticed any difference between the two and MDS operation.

In fact, I hardly ever have my MDS enabled anymore because I never saw any real fuel mileage improvements.

If it were me, I would go with the rear end that's going to suit your needs the most, but the 3. 92 is definitely recommended ;-)

Sent from my Nexus 6 using Tapatalk

 

[hemihustlin]

I dont have the 8 speed but i do have 3.92s and ive had the mds kick on at speeds as high as 140 kmh with very light throttle an a slight downhill incline. I can also keep it in mds inbetween 120-130 kmh for sometimes a suprising amount of time with throttle feathering and a good tailwind.

 

[Q68]

Just push the Tow Haul button on the dash when you test drive them both. It will disable the MDS and you can get a true feel of the trucks, one vs. the other.

It really just depends on what you want, fuel economy or a little more in the seat of your pants. The 8 speed 3.92 is quite impressive in my opinion. I have never driven a 3.21 so I dont have a fair measure though.

Even with the 3.92 I'm avg 23mpg on the freeway.

Good to hear you do that well on the highway, coming out of a 2004 Infiniti G35 that gets 20-22 combined (on 93 octane) I figure I can live with the economy hit if I can manage about 16-18 in 50/50 mixed city/highway.

What do you run for cruising speed to get that mpg, and do you have any idea what the engine is turning at 60MPH?

 

[Copperhead7310]

I actually run a mix of 87 and 93 octane in my truck, 100% gas. Running pure 93 the truck is really smooth but the mpg goes down about .5. If I mix it, it's better than pure 87. I think the truck really likes about 89 octane, but I dont have access to 89 100% around SE TN.

Freeway speeds 65-70ish, the truck is around 1800 RPM and is completely content. It is just barely humming. Crisp throttle response when necessary too.

 

[Bird]

I've got the 3.21 with a 6-speed. I noticed for to accelerate at highway speeds it usually drops a gear. I do notice that MDS is active at speeds below 80mph or when you're not on the throttle.

I would also like to note that in my experience that for MDS to be active it's got to be a straight, level road. I've found that whenever a road gets the slightest decline it disables. When I first got my truck I treated her like a queen; usually set cruse control at 72mph and let MDS run for the majority of the trip to work (40 mile commute). However I usually haul ass to work anymore (80+) so MDS is not active, and I don't notice a huge fuel savings. Maybe 1 or 2 MPG.

 

[MyOhFive]

I have the 8 and 3.92s, but I test drove an 8 and 3.21 truck as well. Get the 3.92 hands down, yes it hauls harder off the line, but it also ups your towing capacity, and yes it will stay in MDS longer. a 3.21 truck will downshift in the slightest grade or acceleration on highway, whereas my 3.92 I can actually do a light accel on highway and it'll stay in 4cyl. (You can hear it).

The only time a 3.21 would ever really outshine a 3.92 is on loooong road trips. And realistically... it's a truck, is 50km less per tank going to kill you? Prob not...

 

[Sportznut]

The MDS is so seamless I can't tell when it is activated. is there a light? is it the Eco light?

 

[The Crow 1994]

The MDS is so seamless I can't tell when it is activated. is there a light? is it the Eco light?

No. It is not the ECO light.

 

[rexhasahemi]

My truck came with the 3.21 gears but I swapped them out for 3.92's. MDS does stay activated longer and over more varied terrain vs 3.21. I do mostly city driving and I actually get about 1 mpg more now.

 

[loveracing1988]

Just push the Tow Haul button on the dash when you test drive them both. It will disable the MDS and you can get a true feel of the trucks, one vs. the other.

It really just depends on what you want, fuel economy or a little more in the seat of your pants. The 8 speed 3.92 is quite impressive in my opinion. I have never driven a 3.21 so I dont have a fair measure though.

Even with the 3.92 I'm avg 23mpg on the freeway.

Tow haul doesn't disable mds with the 8 speed.

 

[Q68]

My truck came with the 3.21 gears but I swapped them out for 3.92's. MDS does stay activated longer and over more varied terrain vs 3.21. I do mostly city driving and I actually get about 1 mpg more now.

Exactly the information I was looking for. Thanks!

 

[skinsno1]

I had a 13 quad 5.7 with 3.55 rear and now have a 15 5.7 with 3.21 and the 13 felt so much quicker with the 3.55. but gas mileage is def better in the 15 with 3.21. highway speeds seem the sam though, its just the from the start speed.

 

[Diller]

No. It is not the ECO light.

It is in the 2013. You can turn the light on or off in the EVIC.


I have the 3.21s and the forums make it seem like they turn the truck into a Prius. I drive 25-30K miles per year so 1 or 2mpgs does make a difference for me. I also tow an open car trailer with a car a lot throughout the year and don't ever really think I need any more gear. It's really not the end of the world.

 

[Andy578]

MDS really only seems to kick on during light throttle and i think maybe while coasting. during acceleration it's almost never on unless you're extremely light on the throttle. as an example if i'm doing 90km/h and for whatever reason drop down to say 85km/h when i try to get back up to 90 i have to really feather the throttle or it'll instantly deactivate MDS

i've heard some say they've seen MDS activate at pretty high speeds but i've never noticed it come on over about 110km/h unless i coast for a bit. of course that doesn't mean it's not coming on just that i simply haven't noticed it so it may or may not be doing it's thing. i really wish they gave us some sort of indicator it would make things so much easier and really help people maximise fuel economy

 

[The Crow 1994]

It is in the 2013. You can turn the light on or off in the EVIC.

There are guys with tuners that have their MDS disabled and their Eco light still illuminates. I have a '13, too and my Eco light comes on despite my truck not running in MDS mode at that time. The light on means you are driving in a fuel efficient manner, not indicative of how many cylinders are being used.

 

[loveracing1988]

There are guys with tuners that have their MDS disabled and their Eco light still illuminates. I have a '13, too and my Eco light comes on despite my truck not running in MDS mode at that time. The light on means you are driving in a fuel efficient manner, not indicative of how many cylinders are being used.

Most times though if the Eco light is on you are in mds mode. Using the buttons to manually shift into 8th gear mds is off and the Eco mode light never comes on.