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HEMI ROCKS!
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Discussion Starter #1
Do they help a 4.7 with an auto get better fuel mileage? And obviously give more snap on the takeoff? Was thinking about putting these in my 04. And how do you fix the calabration of the odometer?
 

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Big Blue 1500 said:
Do they help a 4.7 with an auto get better fuel mileage? And obviously give more snap on the takeoff? Was thinking about putting these in my 04. And how do you fix the calabration of the odometer?
The speedo won't change. You will get better gas mileage and your takeoffs will be fun. :)

Nick and a few others have them on their 4.7's and they're pleased.

Sil
 

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All I can say is I love them, power & mileage is great.
 

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HEMI ROCKS!
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Discussion Starter #4
Thanks guys for the replies. Can anyone tell the jump in fuel mileage that they have gotten by changing them thanks.
 

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Mine was about 1-2.5miles depending on how I drive. But I did come from 3.55 gear ratio
 

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HEMI ROCKS!
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Discussion Starter #6
WOW, that much cool I have 3.92's now hoping I can get that much after the switch. probley won't put them in until spring time. Thanks again for your help
 

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No problem, let us know how you like them when you get them installed.
 

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HEMI ROCKS!
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Discussion Starter #8
Thanks will do.
 

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Very Niiiice! :)
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MikeHTally said:
Just don't expect an improvement on the highway. That mileage will suffer.
I assume that is from personal experience, not something you heard from someone once... :thatfunny

I have a 4.7L QC with 4.56's and they are UNBELIEVABLE!! They helped quite a bit in BOTH my city and highway driving... does it run at considerably HIGHER RPMS... yes, but is it LESS strain on the engine when crusing... YES! Less strain = better mileage. Simple.

I went from 3.55 OPEN differential to 4.56's with an Auburn LSD and gained MPG on both city and highway... more with city driving, but still pretty good gains on both.

one thing to consider... with my 4.56's and 245/70/17's, at 80mph, i was running at about 3000rpms... 70 was about 2400rpms.

another thing to note. I have 20" rims and 325/60/20's on my truck with a 6" lift. At the drag strip a few weeks ago, i was able to BEAT (consistantly) another member with a QC 4.7L but mostly stock... so even with the nearly 36" tires weighing (in combination with my rims) 120lbs a pop, and a 6" lift which makes me LESS aerodynamic, I was still able to run STOCK times with just the 4.56's, lsd, intake and exhaust...

4.56's are my #1 mod to date, hands down!

-David
 

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How can a 30% increase in RPM be less strain on the engine? At steady-state speed, fewer RPM is less gas used. Simple logic. This has been a factor for years. My truck with 3.55 gears is turning < 2,000 rpm on the interstate. Granted, it doesn't have the low-end grunt of 4.56s, but it has enough, plus it gets a pinch under 18 mpg at 80.
 

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Very Niiiice! :)
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Are you taking the overdrive into consideration?

First off, well i guess its too late for that, I wanna say I know next to nothing when it comes to anything mechanical about our trucks, or even auto's in general, but what i DO know is what i have seen...

I have seen me and a fellow member (same as mentioned before) with a stock 4.7L QC with 3.55 gears and open diff, and mine with 4.56's with LSD, we were BOTH crusing at 70mph, BOTH hitting the headwind 1/2 the time, BOTH drafting the other 1/2, taking the SAME EXACT route... and one of us got 22+MPG, one of us got high 19's, low 20's... can you guess which is which??

Riddle me this...4.56's DO run at a higher RPM on the highway, but still get better mileage... how? ;)

-David
 

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SRT-8 Dakota
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Im interested in this as well I was thinking about going to 4.10's in my dakota but not sure if that or 4.56's would be better. At some point im thinking of doing a hemi swap. I also do quite a bit of highway driving. want to see whats out there. not trying to highjack
 

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Im with Schawks73, I drive highway 80% of the time and my mileage has increased(80mph and under)
 

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schawks73 said:
Are you taking the overdrive into consideration?

Riddle me this...4.56's DO run at a higher RPM on the highway, but still get better mileage... how? ;)

-David
Very likely both trucks have over drive, with a final drive ratio of 0.69:1 with the 46RE or 0.50:1 with the 545RE, so that's cancelled out. If one of the trucks you mention has the 545 and the other has the 46, that would account for a difference. I don't think the 545 is used with the 4.7 engine, but I don't know for sure.
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In either case, each engine slurps up x number of cc's air/fuel mixture for every revolution of the crank shaft. A mile covered at 2,000 rpm will use 1/3 less gas than that mile covered at 3,000 rpm. Relatively easy to prove with access to a dynamometer (this would remove the wind resistance variable), but from what I've read about the cost of a dyno run, tain't worth it. Sorry, but it just isn't possible under controlled conditions for a higher rpm to use less gas. That's why these thirsty Hemis have the lower final drive ratio. It helps them get almost as good mileage as the 4.7.
 

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Sorry I cant help with the details on tranny,etc or I would. I'm just giving my results with them
 

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SRT-8 Dakota
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my dak has the 545rfe so i wouldnt be surprised if it does KRS on your passenger side of the truck under the hood look at your PCM if you have 4 plugs on it then you have the 545rfe
 

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i love them, my mpg improved a bit, i know it did not get worse.
 

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03DakStampede said:
my dak has the 545rfe so i wouldnt be surprised if it does KRS on your passenger side of the truck under the hood look at your PCM if you have 4 plugs on it then you have the 545rfe
I will report back in a bit
 

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Just because an engine is turning higher RPM's doesn't mean its using more fuel to do so. For instance, your driving uphill in overdrive at 1000RPM and the truck starts to slow down. You push your gas pedal further down and further to about 1/2 throttle(lets assume the truck doesn't kick out of overdrive for this example). You are just pouring in more gas and getting worse mileage than if you were you kick out of overdrive and turn higher RPM's at only 1/8 throttle. Make sense? Turning higher RPM's means the engine isn't working as hard to move the truck - kind of like how MDS barely ever kicks in because in overdrive with low RPMs you constantly are giving it more gas to keep the vehicle moving.
 
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