Has anyone put a set of 3.43's on a 6.7 Automatic?

I'm interested in optomizing my MPG. . but it's big bucks.

 

Steve, I cannot imagine my rpms any lower on the highway than they are now. I think that if the rpms are too low then you will be out of optimum mileage range the same as if the rpms are too high...
That's just what I think....

 

Agreed. With the auto and 3.73's, I'm only turning 1500 at 65. That in comparison to my 06 manual that would have been 2100. If you get too low in the rpm's the computer will actually pump fuel into the engine to bring it up. Somewhere between 1600-1800 is where I get my best mileage.


Chris

 

I think the rpms are to low with the 3.73s. As stated above my best miliage is from 1600-1800. With the egr unplugged and I drive it right Im seeing nearly 20mpg. I personally want to get 4.10s, I think I would get better non highway miliage. Now if you constantly drive above 80mph you may have something there.

 

Your best RPM is 1650.

Your ECM will NOT pump extra fuel into the system. That is over fueling. These ECMS are designed NOT to over fuel.

 

No Huh? Nobody has any ideas?

 

Check around for one of the 6 speed manual guys that wants to swap. Don't the new manual tranny trucks come with them? I kind of agree with Airgrabber that it will probably hurt you unless you are constantly on the highway at 80mph+. I think you will be lugging the engine alot driving in-town unless you always shift manually...

 

Yeah Mark. . My instincts agree now. But on the other hand, we've added so much power to these with the gadgets, it's hard to say. The factory rep says they are doing it, but it's an unofficial test. Probably 300,000 of 80+ circles

 

Mark???

I know mine lugs the engine if I come up on a hill in town and I'm just cruising in 6th gear. Usually I run around in 5th gear in town just to keep that from happening. I would be more inclined to go to 4:10s as somebody already mentioned....

 

It's not an overfueling situation. The Cummins ECM has parameters based on load that must be met to maintain it's target RPM. Yes it does add fuel to meet this target.

Take a ride in a manual truck get it into second and take a slow curve. The vehicle will accelerate on it's own to prevent stalling out. It's a built in idle circuit in the software. If you had owned a manual, you'd see. One of my favorite things to do was to be rolling along at speed, throw it into a high gear and mat it. What happened? It rolled coal. Why? Because the ECM needed to ADD fuel to compensate for the low RPM's and the load requested by the go pedal.

Chris

 

Also you will notice, that most of the problems going on are from lugging the trucks at too low an RPM. If the ECM wasn't adding fuel here, there wouldn't be an abundance of soot and unburnt fuel clogging the DPF's. Heck even my auto truck lugs. If it goes into 5th or 6th at 40 mph in town, you can feel the truck trying to spin the turbo to get up above 1500 RPM. It does this by adding fuel. Not to rant, but I felt this needed explaining more.


Chris

 

That's a different scenareo than what we were talking about. What you are talking about is lugging at low speeds. Hard to do on an automatic. Low speed lugging can crack rings on a diesel too.

They were referring to lugging the engine when running up hills at higher speeds. I am pretty sure the ECU won't accelerate at those speeds.

But it is good input. Thanks

 

[QUOTE=Polaraco;2259930] What you are talking about is lugging at low speeds. Hard to do on an automatic. /QUOTE]

I almost never drive this truck in "full" auto. Its always in manual. (Yes I miss my 5600) ///////you can lug these things down to around 1000rpm or so pretty easy. Then it will shift on its own. But only after it tries to give some extra juice when you ask it to go. It will try to keep it in sixth and you'll notice a little extra when it tries to get it goin again. Thats the only time I notice any add fuel by the truck. As far as rpms with the 3.73, most roads that I drive on, I usually run about 50-60 ( non highway ) The motor seems to be working a bit harder ( more fuel ) tring to keep my speed where I want it in 6th. And even 5th. Dont get me wrong, there more than enough there to keep it moving well, but going to a 3.42 you'd never shift out of 4th gear under 60. Probably work great without a speed limiter at the salt flats.

 

If I lug mine for too long it will downshift automatically and it doesn't take too much lugging to make that happen....