3.92 rear failed,new 3.55, new tune? BAD Fuel Milage! - DodgeForum.com

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3.92 rear failed,new 3.55, new tune? BAD Fuel Milage!

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  #1  
Old 03-24-2010, 03:20 AM
Pilotdriver
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Angry 3.92 rear failed,new 3.55, new tune? BAD Fuel Milage!

Help and a beware, 2004 Ram 1500 A/T 2WD posi rearend failed at 74000 miles, took out entire center section. Had 3.92 gears, Truck is babied and never tows. So i went with the stock 3.55 gears, fuel mileage has dropped over 2.2 mpg in town and about 2 on the highway. Hemi doesn't care, truck drives fine doesn't hunt for gears etc. About a 250 rpm drop in overdrive at 65 mpg.

Does the PCM need to be updated to show the different rearend? Can i tune thru the pcm for fuel mileage. This has me very upset to where these gears may have to go and another expensive set of 3.92's goes back in.

Any ideas, I'm old school and didn't expect any huge improvement with the 3.55's but never saw this coming.

Thx
 
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Old 03-24-2010, 03:30 AM
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What size wheels you got?
If they're 20's or bigger, then the change to 3:55's will do that to your MPG's.
Nothing to do with the PCM. It's just the truck doesn't have the mechanical advantage of the 3:92's.
That's why everyone that can, has changed over to 4:56's.
I unfortunately run 3:55's in my Hemi and get crap MPG's. I did a bit of digging and found that my truck originally came from the factory with 17's and the factory usually, unlees the customer specified, stuck in 3:55 gears as stock for this option. But I guess that someone in the past liked the look of the 20" petals and stuck those on, but didn't take into account the heavier weight of the wheels and tyre combo and that's what I'm left with.
When I do get together enough money, I will be putting 4:55's into my truck as I'm assured it will really wake it up and give me better mpg's.
Al.
 
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Old 03-24-2010, 03:55 AM
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the truck has the 17's , all stock from day one including tire and rim sizes, I have the tow pkg as an option so that's why I had the 3.92's

Was getting a consistant 15 mpg town and 19-20 on the hiway. i've computed the mileage both the old fashioned way as well as relying on the computer in the truck, same numbers, all bad!
 
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Old 03-24-2010, 04:01 AM
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Originally Posted by Pilotdriver View Post
Help and a beware, 2004 Ram 1500 A/T 2WD posi rearend failed at 74000 miles, took out entire center section. Had 3.92 gears, Truck is babied and never tows. So i went with the stock 3.55 gears, fuel mileage has dropped over 2.2 mpg in town and about 2 on the highway. Hemi doesn't care, truck drives fine doesn't hunt for gears etc. About a 250 rpm drop in overdrive at 65 mpg.

Does the PCM need to be updated to show the different rearend? Can i tune thru the pcm for fuel mileage. This has me very upset to where these gears may have to go and another expensive set of 3.92's goes back in.

Any ideas, I'm old school and didn't expect any huge improvement with the 3.55's but never saw this coming.

Thx
Pilotdriver,
I understand your confusion, I too am old school and what is being shown is that our Hemi's like to run at a higher rpm than stock gears provide. Actually increasing mileage under certain conditions such as around town and hilly country. Many folks have installed 4:10 or 4:56 gears and have improved their mileage. Do a search on gearing and reads the threads. You will be surprised. I know I was, but the thing is with our overdrive of .67 the 4:56 computes out to be something like a actual 3:73. And in the old days the 3:73 was really a nice gear to have. Our trucks are too heavy for the tall gears that came from the factory to be nimble and we end up leaning on the gas to make them go hurting our mileage. Do some research, I am sure you will be surprised at what you find, I was. Good luck...
 

Last edited by RedTruck-VA; 03-24-2010 at 05:47 AM.
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Old 03-24-2010, 05:42 AM
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Wow, did you go the wrong way! The only time the 3.55s might and I say might help with fuel economy is on the open highway after you get it up to cruising speed. THIS IS A 5000# TRUCK HERE! Sounds to me like you originally had the "C-Clips" fail (like everyone) and didn't bother to get them babies out of the rear end till they took your ring and pinion (and prolly the spiders too) out to lunch with them, eh?

Gears are not expensive at all, if it was just geared, you wouldn't need an install kit, so at most 3.92 or even 4.10s preferably would be $200 for the ring and pinion and maybe $300 labor, topps...
 

Last edited by HammerZ71; 03-24-2010 at 06:07 AM.
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Old 03-24-2010, 05:52 AM
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Yeppers.........I had the factory 3.21s! I did get great mileage on the highway, but I was always standing on it to get up to speed. I switched to 4.10s and my mileage really did not change much. I may have lost some on the highway, but I get up to speed so much more effortlessly that I gained it back in town.

Its weird to me that Dodge seems to do gears based on the wheel size. In reality, the 20s vs the 17s are about the same rolling diameter. The 20s are about 33" whereas the 17s are almost 32". I wouldn't think that an inch or so would make that much difference.
 
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Old 03-24-2010, 08:23 AM
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Originally Posted by Pilotdriver View Post
the truck has the 17's , all stock from day one including tire and rim sizes, I have the tow pkg as an option so that's why I had the 3.92's

Was getting a consistant 15 mpg town and 19-20 on the hiway. i've computed the mileage both the old fashioned way as well as relying on the computer in the truck, same numbers, all bad!
ALL BAD????? are you kidding? i would kill for that. i have 17's with 3.55 and i am LUCKY to get 13.5 on the highway!!!!!! i am averaging overall 12 mpg. and i only compute by hand. don't trust that onboard stuff. never been right for me.

on a sidenote, truck is 95% babied. never tows (since i have nothing to tow currently but will) and hardly ever put my foot into it since i really never need to. i sooooooooo need to get 4.56s.
 

Last edited by MOusafLRT03; 03-24-2010 at 08:27 AM.
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Old 03-24-2010, 11:30 AM
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Originally Posted by MOusafLRT03 View Post
ALL BAD????? are you kidding? i would kill for that. i have 17's with 3.55 and i am LUCKY to get 13.5 on the highway!!!!!! i am averaging overall 12 mpg. and i only compute by hand. don't trust that onboard stuff. never been right for me.

on a sidenote, truck is 95% babied. never tows (since i have nothing to tow currently but will) and hardly ever put my foot into it since i really never need to. i sooooooooo need to get 4.56s.
The OP has a much more fuel efficient engine. It doesn't say whether he has a 4.7 or Hemi in the post, but in either case the design is 20+ years newer than that venerable old LA360 in your truck. A Hemi will produce similar torque and 100 more HP while still getting 25-35% better fuel economy.
Not that I'm bashing your engine, that thing is a tank and with marginal care (and a decent plenum gasket) should last 200,000+ miles. Get a Hughes solution for the plenum problem so it will handle an increase in HP, add a Superchips and 4.56s and you'll have a beast on your hands...
 

Last edited by HammerZ71; 03-24-2010 at 11:33 AM.
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  #9  
Old 03-25-2010, 04:06 AM
Pilotdriver
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Your right C clip took out the posi and ring and pinion, plus the bearing spun on the pinion. truck has the 5.7 Hemi. I drive a lot of miles and with gas at $3 a gal I can pay to have the 3.92's put back in with fuel savings in less than 6 months. Any thoughts on just going with the 4.10. Don't tow but do carry some heavy gear in the bed regularly.
I guess I just figured that the Hemi power/torque curve could handle the 3.55's without taking such a beating.
 
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Old 03-25-2010, 04:15 AM
rickof08
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Id go with the 4.10's. You cant beat mechanical advantage.
 
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