3.21 vs. 3.92 gear ratio - Ram 1500

[The Critic]

Critic, sorry if I missed it, but what factory tires came with your Ram and what do you air them up to?

I'm hand calculating 21mpg average with my 2019 Ram 1500 classic, w/ hemi and 3.21s. My truck has the 17" wheels with GY Wrangler SRAs that I have aired up to ~41psig.

Goodyear Wrangler Fortitude HT, 275/65-18. They are inflated to 36-psi per the placard.

I think I arguably have the most efficient combo - 2wd, 3.21, eTorque and 18" wheels with LRR tires. A lot of trucks are equipped with larger tires which will only negatively impact the fuel economy.

 

[14Accent]

Lots of good info in this thread, I've read it purely out of curiosity.

My .02: is it really worth getting the 3.92 with the electronic locker? Personally, I'd rather have the 3.21 with a standard posi unit. Less electronics is better in my book. I've only ever driven one car with a posi, as at 31 I haven't had much experience with owning RWD vehicles. I will say, my '03 Mercury Marauder with a 3.55 posi unit managed to survive 3 MN winters with no more than the weigh of my subwoofers in the trunk.

I'll throw in a vote for the 3.21's with the regular posi.

 

[supton]

IIRC the transmission has an input torque limit (well all transmissions do) and as a result full engine torque isn't allowed in first gear. Thus the 3.92 might have more snap off the line as a result. Once out of 1st or so, then they should start to act similarly--transmission just goes up or down gears as necessary, and the engine is allowed to generate more torque in the other gears.

Anyhow. Electronic locker? Is that a system where it pulls brake lines to limit slip, or is it a true locker? I guess I could see either way. A posi might be better off road, but on road the brake line method is likely just fine in snow--lock both wheels together and one is more apt to be sideways (always fun when unintentional)--although maybe that is overblown--my open rear diff truck has gotten sideways at inopportune times, maybe it'd be no worse with a posi. hmm, I think I might have argued myself into the posi camp with that as I never thought the brake line method did anything for me in my truck (usual winter driving pre-drive routine has me turning off TC so I don't get stuck).

 

[JTK]

Goodyear Wrangler Fortitude HT, 275/65-18. They are inflated to 36-psi per the placard.

I think I arguably have the most efficient combo - 2wd, 3.21, eTorque and 18" wheels with LRR tires. A lot of trucks are equipped with larger tires which will only negatively impact the fuel economy.

Indeed you would think it's the most fuel efficient combo you've got there.

Almost all of the guys I work with, with late model Ram 1500s have gone to load rated LT tires and per them, they saw a definite loss of fuel economy with them. Most of them claim to get around 16mpg day/day and possibly 21-ish on the highway. Funny thing is, many of them don't tow anything! They wanted the LT tires because they thought they'd last longer. Not the case.

I prefer the 17's that my 1500 classic SLT has. They don't look as sporty, but the ride is cushy and winter time performance is better than the 20's most of the classics come with. I think your 18's are the best of both worlds.

 

[hank2]

With the many speed transmissions today with very tall top gears and big tires/wheels I would want the 3.92, if those are the choices. I used to have a Silverado with a 4.8 and 4 speed auto. It had a 3.45 differential and 32 in. diameter tires. I would not have wanted anything taller geared. It did well, but a 3.73 would have been perfect. I believe a 3.2 was standard and I would not have wanted that with the tire sizes.

 

[Ws6]

I had 3.73's in my Jeep with the HEMI. Loved it. Still go 18-20mpg on road trips.

 

[dlundblad]

These are the only 2 options? Seems like a very wide spread. No middle of the road gears like 3.55 or 3.73?

I’d take the lower gears personally. What’s the RPM difference at 60 MPH? Heck, it might even get nearly identical mileage while in town especially if one has a lead foot.

 

[Gene K]

I used to hang out on the RAM forums and while I didn't have a Hemi the guys claimed they did better with the 3.92 Gears.

The claim was the 3.92 is able to maintain 4-Cylinder Mode more so gets better mpg with real world highway speeds.

 

[gfh77665]

These are the only 2 options? Seems like a very wide spread. No middle of the road gears like 3.55 or 3.73?

As I mentioned above, the RAM does come with the 3.55 option. It really is the best of both worlds. With my 3.6L Pentastar V6,

3.21 gears = 4970 lbs towing
3.55 gears = 7260 lbs towing, with almost identical MPG.

 

[The Critic]

As I mentioned above, the RAM does come with the 3.55 option. It really is the best of both worlds. With my 3.6L Pentastar V6,

3.21 gears = 4970 lbs towing
3.55 gears = 7260 lbs towing, with almost identical MPG.

I think the 3.55 option is only available with the 3.6. HEMI's are only available with the 3.21 and the 3.92.

 

[supton]

I think the 3.55 option is only available with the 3.6. HEMI's are only available with the 3.21 and the 3.92.

I always thought that a bit odd. Not sure if they use the same axle? I mean, if the Pentastar has a lighter duty rear end then perhaps they just didn't want to make a pile of different ratios, since each gearset would have to be designed up.

I don't feel like looking up the numbers but IIRC the jump from 3.21 to 3.92 was on the order of the jump in any of the trans gear ratios. As in, driving in 6th gear with 3.21 was like driving in 7th with the 3.92. That would mean the 3.55 was splitting the difference--at any given speed, given the 8 gear ratios had, the 3.21 vs 3.92 was simply a wash--the trans could just change a gear and have the same engine rpm regardless. But 3.55's, being between, could actually tailor engine rpm a bit more, at any given fixed speed. [Ignoring takeoff ratio for the moment.]

 

[meep]

Interesting discussion. One thing that is seldom mentioned is the lower ratios are favorable for not only mpg but also for axle temps. OTR trucks run surprisingly lower ratios for what they do specifically to increase efficiency and manage temps. With our smaller (by comparison) trucks, yes, the deeper gearing helps our “little“ trucks pull heavier loads.... I loved the 4:10s in my gen1 tundra for towing, though it became incredibly hot in the hills... like 190F hot... while the trans would be between 160-180, but in an optimized-for-towing application taller gears have a place. Having learned that, I’m happy with a lower ratio if the rest of the driveline can handle it.

m

 

[JTK]

I think the 3.55 option is only available with the 3.6. HEMI's are only available with the 3.21 and the 3.92.

I believe 3.55s were available on Hemi Rams with the 6spd auto. I've never seen it on 8spds like you say.

To me the 3.21 vs 3.55 is the oddest thing on the Pentastar powered Ram 1500s. When my brother purchased his 2019 1500 classic express, pentastar QC, 4x4 new, the 3.55 gearing was something like a $45 stand alone option per the MSRP window sticker. The dealer he bought from happened to have 2-3 on the lot with this option that he specifically wanted per my recommendation given the higher tow weight specs.

There was no noticeable fuel economy difference between his with 3.55s and my 2017 with 3.21s.

 

[supton]

Interesting discussion. One thing that is seldom mentioned is the lower ratios are favorable for not only mpg but also for axle temps. OTR trucks run surprisingly lower ratios for what they do specifically to increase efficiency and manage temps. With our smaller (by comparison) trucks, yes, the deeper gearing helps our “little“ trucks pull heavier loads.... I loved the 4:10s in my gen1 tundra for towing, though it became incredibly hot in the hills... like 190F hot... while the trans would be between 160-180, but in an optimized-for-towing application taller gears have a place. Having learned that, I’m happy with a lower ratio if the rest of the driveline can handle it.

m

I didn't even think of that... more turns of the pinion to get the same axle shaft speed. More turns, more wiping action in the gears, so more frictional losses. But input torque would be lower so I'd think it'd be a net sum zero game... apparently not, the big rigs would know for sure. Very interesting.

 

[OppositeLocK]

I have an EcoDiesel with a 3.92 ratio. Wouldn't have it any other way. I get around 24 mpgs (combined) but I don't tow anything.

The diesel is slow but extremely torquey. The shorter gearing really helps and I think it matches the ZF 8 speed nicely.

 

[meep]

I didn't even think of that... more turns of the pinion to get the same axle shaft speed. More turns, more wiping action in the gears, so more frictional losses. But input torque would be lower so I'd think it'd be a net sum zero game... apparently not, the big rigs would know for sure. Very interesting.

yeah I read about this a few years back. We all talk about ATF temps but what I learned is the OTR guys worry about the transmission a little less if it is actively cooled, whereas the axles are always passively cooled. So they were saying that if you wanted to instrument a consumer vehicle for tow duty, to instead mount temp sensors on the axle. The ecu/tcu will let you know if the trans is beyond acceptable limits.

i sold that truck, and with the new one I just haven’t felt like cutting holes in it to mount gauges. that, and for real, I don’t hammer it when towing and should be fine anyway. Priorities changing as tome goes on...

m

 

[Skippy722]

yeah I read about this a few years back. We all talk about ATF temps but what I learned is the OTR guys worry about the transmission a little less if it is actively cooled, whereas the axles are always passively cooled. So they were saying that if you wanted to instrument a consumer vehicle for tow duty, to instead mount temp sensors on the axle. The ecu/tcu will let you know if the trans is beyond acceptable limits.

i sold that truck, and with the new one I just haven’t felt like cutting holes in it to mount gauges. that, and for real, I don’t hammer it when towing and should be fine anyway. Priorities changing as tome goes on...

m

It seems we’re slowly working our way towards active cooling of rear diffs. Certain 5th gen Ram’s come with a “thermal axle” upgrade. It’s more for heating up the rear diff to eek out a tiny bit more mpg, but it should also help keep it cooler under heavy loads.

 

[The Critic]

It seems we’re slowly working our way towards active cooling of rear diffs. Certain 5th gen Ram’s come with a “thermal axle” upgrade. It’s more for heating up the rear diff to eek out a tiny bit more mpg, but it should also help keep it cooler under heavy loads.

Just realized that my 4X2 5.7L eTorque has the "thermal axle" option:

It looks like the "Thermal Axle" is only on the 4X2 eTorque models. The 4X4 does not have it.

 

[supton]

Hmm, not sure how much cooling you could get from the rear axle? Oh wait... really long lines, maybe more than I think. And it's not like it needs that much either, if I had to guess.

Ya know, for years I've pointed out that leaving the car at idle in winter to warm up the engine does nothing for the rest of the driveline. That's not as true as it used to be! I bet this could have a big impact on cold weather losses. Start getting that gear lube warming up. Well, don't idle, I don't mean that, but as the coolant heats up fast, it should really help the rear diff lube.

 

[meep]

I’d be a fan for actively controlling temps. That’s pretty slick. I believe nascar actively cools them with a belt off the pinion shaft driving a pump and external radiator, but a simple coolant loop through the bottom of the case would be easy enuf to do.