I am considering a gear change, 4.10's to 3.42's. I can do all the work myself in the garage and the parts seem to be available from a couple places now. One quote with bearings and such was $950 for both front and rear and included a ring gear spacer for the front so I wouldn't have to find another carrier.

I run an NV5600 and 285/70-17's and 4.10's now. I drive 135 miles a day to work, all interstate with a little rush hour thrown in. My rpm will drop from about 2450 at 75mph to about 2050 (about 16.5%). 60mph has always been the sweet spot and that runs 1950rpm as is. I used to drive 60mph to my previous employer, 40 miles each way, and I got dead on 21.0 mpg every time. Right now if I can hold 70mph I get about 16-17mpg.

So I'm hoping/guessing that I can get better mileage but that depends on how much of it is aerodynamics and how much is engine rpm. I am pretty sure I will save wear and tear on the tranny, diffs, ujoints, and engine so that's worth something since I plan on having this truck as long as I can.

Has anyone done this yet? I can't seem to find any good data that speaks to the difference between 3.73 and 4.10 555's with 6 speeds. Any ideas? I'm also ready to spend the dough on a cam/lifters and a new turbo. I'm also running a BDTD and it hasn't hurt mileage unless I'm playing. How would these gears affect the cam and turbo selection? I tow a fair amount, but never really over 9k trailer weight.

I guess the way I see it I never use 1st gear on the transmission as it is and if the gear change hurts the top end more than I would like I could always shift into 5th and be back where I was with 4.10's.

Ideas? Opinions?

Andy

 

I didnt know 3.42s were available for AAMs yet, but if they are, sweet!
If I had a 6spd, id be all over it, but with my auto I cant loose more gear than I have with the 35s.

 

Yeah, I'd like to take those 4.10's off your hands and throw 'em in my truck.

 

I would also be interested in the 4.10 if you decide not to keep them.

 

i had an '01 with 3.55's and i hated them with a passion. i did a lot of cruising at 60 to 80 mph and the best mpg i ever saw was 17. the only mods on that truck was a 4" straight pipe and CAI.

 

JMHO, but unless you have done gears before, I would have someone who not only knows what they are doing, but also who does it on a regular basis. It's not rocket science, but if they are not set up right, you will be hunting for a new set- you don't want that. So best bet is to have a knowledgeable individual beside you or spend the money to have someone do if for you- there's no shame in that. Then you might not have to change them more than once. Like I said, JMO.

 

Actually I have set up gears before and these axles are about as easy as they get, other than the weight of the components! Not a problem for me

Andy

 

Have you thought about a US Gear or GearDrive? That would allow you to keep the gears you have but lower your RPM when cruising.

I believe you can get into a gear reduction unit for about $1500.

I have not fully checked the units out, I have been meaning to.

 

Running a whole separate gearbox will be a lot less efficient than changing to a different ratio. It would lower the engine RPM but raise the mechanical loss. Don't forget that each overdrive gear is less efficient than a straight through box and adding another OD behind the one you already have just makes matters worse. Only benefit would be if your ratio was way off or you needed more selections. But the change in RPM may not raise the efficiency of the engine enough to make up for ODs losses.

With the 3:42s and a little slower driving, like on secondary roads where the speeds are lower. He could drive in 5th gear (direct) and be really efficient. On the highway empty, the 3:42s might be the perfect ratio in 6th. Gearing is not just about being higher it's about being appropriate. If you're winding it, it hurts you. If you are lugging it, it hurts you. If you are running extra gear sets, it hurts you. If you raise the ratio by adding enormous tires with huge wind resistance, it hurts you even though the engine slows down. The OD has internal pumping and gear losses, so before you get any benefit from slowing the engine you have to first make up those losses.

I say do some accurate MPG testing with the 4:10s then do the same with the 3:42s and see waht's up. Around town probably no difference, highway empty, a little better. And a lot less annoying with lower RPM.


John

 

If it were me, then I would go ahead and do it- 135 miles a day to work is a pretty decent amount of mileage. You'll probably be happier. I'm pretty happy with my setup in the 3.73/G56-it's like the NV5600 w/4.10's. I like the get up and go, if I had your trans I would probably want the 4.10's just the same. It's all personal preference, and what works best for each person.

 

If my math is correct (18.75 centavos per mile at $3.00/gal, 16mpg) then it will take ~5500 miles to pay for $975 worth of gear change if it's worth 1mpg. That's about 8 weeks of just driving to work and back for me.

I wonder if swapping the front axle hubs to part timers is really worth an MPG or two? I wonder if taking the driveshaft out would show any improvement? Kind of a proof-of-concept I guess.

Andy

EDIT: Yeah the math is way wrong. It would take 88,000 miles to pay $975 back if it's a 1mpg improvement. Not sure what I was thinking.

 

My 04.5' has 3.73's with a NV5600 and i turn about 2050-2075rpm at 70 mph. I am always reaching to grab 7th gear! haha I always wondered how it would be with 3.55's like the 2nd gen ctd's. I would like to get down to about 1800-1850 rpm at 70mph. Who knows, just swap in a roadranger 9 speed and have it all!!! haha its nice to dream.
ez

 

I had two Ferds with locking front hubs. I usually ran unlocked of course. Once in a while I would lock them and run around for 200 - 300 miles to re -lube all the components, never really noticed any loss of economy. The only nice thing about having them was I could leave them unlocked and use 4 low when backing the trailer up in tight spots with out worrying about binding up the drive line.

Where have you found the 3.42 gears for the AAM axles? Do they have them for the 11.5 rear and the 9.25 fronts both? I am seriously considering them as well.

 

Nope - your gears are still spinning. W/ hubs you stop the half shafts from spining.

 

Go to http://www.justdifferentials.com/ to find the 3:42 gears.