Eco Hubs 3.90 or 3.07 for Best Fuel mileage

[Keith Knight]

Down to the wire, as I’ve started this project of installing Eco Hubs, replacing front axle housing seals, and pinion seals. My plan is / was / I’m not sure to remove my 3.07 gears and put the original 3.90 gears back in.
I’m second guessing my decision, based on a good friend driving a 2007 M1078 with 3.90 gears and eco hubs weighing in at 26,500lbs, converted into an expedition vehicle. He just finished a 10-12 thousand mile trip out west and back. The best mileage he got was 8 mpg on just one of his many fill ups.
Chris another fellow member here on I believe a 2003 FMTV 6x6 converted into an expedition vehicle running eco hubs but with 3.07 gears and weighing about the same. He has reported getting up to 9 mpg.
My top speed has been 60 mph on the interstate roads over the last 24,000 miles but typically on the smaller side roads averaging 55 mph always getting 6.5 mpg.
Looking at the chart from another post it looks like with eco hubs and 3.90 gears, cruising at 60 mph it would be running in 6th gear just under 1600 rpm’s.
With eco hubs and 3.07 gears, cruising at 60 mph would be 5th gear at 1600 rpm’s.
Just talking fuel mileage what would be better?
Pros and Cons?

 

[GeneralDisorder]

Probably very little difference in practice. Peak torque should be peak efficiency so the closer you are the better.

Mileage depend on many factors. What was their speed? What was their tire pressure? Average altitude? Average temperature? Wind resistance? It can be pretty complicated.

My truck with 3.90's got 10.2 mpg on a test run when we first installed ECO hubs. 55 mph, sea level with tires at 90 psi.

 

[Keith Knight]

Forgot to mention it’s a 3126 tuned to 330hp and 27,700 lbs.

Realizing all the unknowns keep it simple and pretend flat ground at sea level and a steady 60 mph with 85-90psi. Optimal conditions.
which should be better?

 

[Ronmar]

3.90 is closer to the ideal, but the 3126/C7 have lower pk torque RPM So either works OK. the difference between the two is only ~5%. You will of course have 5% less final pull force/thrust with the 3.7…

 

[GeneralDisorder]

I'm not sure you would be able to really tell the difference. On perfectly flat ground, running the same RPM.... theoretically the 3.07 would have the advantage in terms of lower driveline speeds for the same road speed but may put you farther away from peak torque. If you adjust your gear/speed to align with peak torque then I think the difference would be so small you wouldn't be able to measure it in practice. I'm not entirely sure how the drivetrain speeds will offset the distance from peak torque RPM.

That said - I wouldn't trade my acceleration with the 3.90's for the very tiny improvement in mpg at one specific speed with 3.07's. I've driven both and my truck is considerably more "sporty" than the 3.07's. I would install 4.44's if I could get them. Which would just put me in 7th gear around that same 60 mph.

 

[Mullaney]

Forgot to mention it’s a 3126 tuned to 330hp and 27,700 lbs.

Realizing all the unknowns keep it simple and pretend flat ground at sea level and a steady 60 mph with 85-90psi. Optimal conditions.
which should be better?

.
Have you run any calculations on a web based chart?

https://www.tremec.com/calculadora.php

 

[Keith Knight]

What is the peak torque RPM for the 3126 tuned to 330hp?

 

[Keith Knight]

I'm not sure you would be able to really tell the difference. On perfectly flat ground, running the same RPM.... theoretically the 3.07 would have the advantage in terms of lower driveline speeds for the same road speed but may put you farther away from peak torque. If you adjust your gear/speed to align with peak torque then I think the difference would be so small you wouldn't be able to measure it in practice. I'm not entirely sure how the drivetrain speeds will offset the distance from peak torque RPM.

That said - I wouldn't trade my acceleration with the 3.90's for the very tiny improvement in mpg at one specific speed with 3.07's. I've driven both and my truck is considerably more "sporty" than the 3.07's. I would install 4.44's if I could get them. Which would just put me in 7th gear around that same 60 mph.

Thanks for the drivers seat perspective on driving both.

 

[Ronmar]

I think the 3126 and C7 both fall in around 1200-1250 RPM PK TQ. From what I can tell the 3116 is a little higher at ~1550...

 

[ckouba]

Chris another fellow member here on I believe a 2003 FMTV 6x6 converted into an expedition vehicle running eco hubs but with 3.07 gears and weighing about the same. He has reported getting up to 9 mpg.

Hey! That's me, and accurate for my experience. The truck is a 2000 M1088A1 with the 3126.

I shift into 6th at 65 MPH, and if I am restrained enough to keep it there, that's when I get my ~9MPG. If I could get that shift to happen at ~60 MPH, I think I think I'd do even better, and that is what I believe Brandon (@Lostchain) and Rick are experiencing. They also weight significantly less and have less drivetrain loss, so they'll naturally do better.

I don't think there's enough delta for me to be motivated enough to swap them back to 3.90's because frankly, for what this rig can do, I am quite satisfied with 9 on the highway. If I blew up a R&P, I would change back without thinking.

My advice to you is to try the set up you already have. It's easy and free. See if you like it. If you find it lacking, revert to 3.90's and try them. I think you'll be happy either way, and since you're a bit lighter than me, you may have a better/easier time under load and be even happier with the 3.07's than I am.

 

[MatthewWBailey]

With eco hubs and 3.07 gears, cruising at 60 mph would be 5th gear at 1600 rpm’s.

I'm running ECO 3.07 at present. Last highway only trip was 400 mile round trip to UT n back. 75-80 mph speed limit maintained. I got 9.5. I was in 6th on flats and downshifted to 5th on highway hills. I believe the torque is 860 max at either HP tune. I think 3.90 would offer better driveability/granularity as there'd be less shifting on highway hills at speed. But I can't see the mpg changing much as the rpm target will still be 1400-1600.

 

[Keith Knight]

So if Chris shifts into 6th at 65mph then with 3.90 it should shift int 6th around 50mph and maybe set the cruise at 55mph for optimal fuel efficiency.

I’m having to pull the front axle shafts out to replace the axle housing seals. I definitely never want to do this again and I have the 3.90 gears as a third member swap. So no setting up the gears. And listening to Chris that if he had a ring and pinion go out he would go back to 3.90. So after listening to everyone I’m going to move forward with the 3.90 gears.

 

[hike]

@GeneralDisorder is correct: 3.90 is closer to ideal with EcoHubs. 4.50's (or 4.44's) would be awesome on these rigs. Seventh gear would show up on the flats.

We have no experience with the 3.07's. On and off road so far we like the way our truck reacts with 3.90's and EcoHubs—

 

[Ronmar]

So if Chris shifts into 6th at 65mph then with 3.90 it should shift int 6th around 50mph and maybe set the cruise at 55mph for optimal fuel efficiency.

I’m having to pull the front axle shafts out to replace the axle housing seals. I definitely never want to do this again and I have the 3.90 gears as a third member swap. So no setting up the gears. And listening to Chris that if he had a ring and pinion go out he would go back to 3.90. So after listening to everyone I’m going to move forward with the 3.90 gears.

Yea you are already doing all the hard part, swapping an already setup pumpkin will only add 30 minutes to the front tops…

 

[NY Tom]

I think the 3126 and C7 both fall in around 1200-1250 RPM PK TQ. From what I can tell the 3116 is a little higher at ~1550...

I know nothing. But I want to know why running at 1200-1250 is not better...why 1600 then?

 

[Ronmar]

Peak tq RPM is also know as peak volumetric efficiency. It is the point where you develop max tq output for the fuel consumed. It is determined by bore, stroke, manifold design and valve timing and design.

now if you can still make enough tq to accomplish the task at a lower RPM, you may see even better economy at a lower RPM. Some diesels are very happy lugging along below the pk tq RPM.

pumping losses increase drastically above pk tq, which is why TQ is typically down by half at pk HP RPM. Half your pk tq is used just spinning the crankshaft at that higher RPM.

 

[hike]

CAT 3126b spec sheet states:

For the best balance of performance and fuel economy, spec axle ratios and tire sizes to obtain: 2000 rpm @ 60 mph (97 km/h) subject to the following: Maximum cruise speed of 65 mph (105 km/h) or below. Maximum recommended engine speed at cruise — 2400 rpm. Minimum recommended engine speed at a cruise speed of 55 mph (89 km/hr) — 1800 rpm.

Peak torque is 1440 rpm—

 

[hike]

Looking at our crawl-ratio sheet showing standard 2:1 wheel hubs, EcoHubs, 3.90's and 3.07's at rpm's for 1440 (peak torque), 2000 (suggested 60mph), and 2400 (suggested max) rpm's:



3.90's hit CAT's suggested "best balance of performance and fuel economy" in 5th gear. Thank you @Xengineguy.

Swapping tires to 40's would bring 7th gear back—

 

[Keith Knight]

Thanks Hike

 

[MatthewWBailey]

Peak tq RPM is also know as peak volumetric efficiency. It is the point where you develop max tq output for the fuel consumed. It is determined by bore, stroke, manifold design and valve timing and design.

now if you can still make enough tq to accomplish the task at a lower RPM, you may see even better economy at a lower RPM. Some diesels are very happy lugging along below the pk tq RPM.

pumping losses increase drastically above pk tq, which is why TQ is typically down by half at pk HP RPM. Half your pk tq is used just spinning the crankshaft at that higher RPM.

From sizing gensets, I think of this as an analysis of Energy output and torque output. Torque is not energy, so it's a 2 part checklist. Enough torque to move the shaft at 1440, checked. Enough energy (Hp) to overcome wind and grade at 55 at 1440, not checked. Hence why after the 330 remap, I can maintain 52 up a 7% grade whereas before at 275, only maintained 43, with same gearing and almost identical torque curve of engine.