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Custom Dodge Ram Performance Mods - Engine - 4.7 Liter V8 Discuss modifying your Dodge Ram with Performance Parts and Accessories!
Factory Spec: 4.7-liter V8 engine - 310 horsepower, 330 lb-ft of torque.


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  #1  
Old 07-17-2009, 02:37 PM
euromade euromade is offline
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Default New to 4.7 RAM - Bigger tires issue

Recently I've purchased an 05 RAM with 285/75 R17 tires. It appears the rims are stock but the tires were bumped up from the OEM size (245 75 17 is on the door sill sticker)

ISSUE: It appears this truck likes to shift down to early keeping the engine in a very low RPM range. This RPM band is way below its torqu curve so I constantly have to up shift and it drives me crazy.

QUESTION: How do I reprogram the computer so it "knows" about the bigger tires and eliminate premature down shifting.

Thanks a buch.
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  #2  
Old 07-19-2009, 03:44 PM
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bigmike19956 bigmike19956 is offline
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you can get a programmer that will allow for speedometer correction to compensate for the oversize tires. this should take care of your problem.
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  #3  
Old 07-20-2009, 11:37 PM
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jayhkr jayhkr is offline
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I would like to know what the standard RPM is suppose to be for this size engine? My 06 has an RPM @60mph of 1600RPM. At idle it is just above 500RPM. Is this normal, not normal. I have the 355 gear which I believe is stock. Any ideas?
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Old 07-21-2009, 03:08 AM
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Quote:
Originally Posted by jayhkr View Post
I would like to know what the standard RPM is suppose to be for this size engine? My 06 has an RPM @60mph of 1600RPM. At idle it is just above 500RPM. Is this normal, not normal. I have the 355 gear which I believe is stock. Any ideas?

OK....First off the 285 tires are killing you because of the 3.55 Gears you have.......Get new 4.10 Gears or get rid of the 285 tires.... I vote for the gears because I bet it looks great with the tires. The programmer will only adjust the computer for the tire size so that the speedo reads correct. The 4.7l is a little underpowered for the 3.55 Gears and the big tires.....4.10 Gears will make a WORLD of difference........
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Old 07-23-2009, 01:08 AM
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AIR RAM AIR RAM is offline
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I say skip the 4.10s and go right to the 4.56's if your tires are 30" or taller... Hands down 4.56's are the mathematically correct gear. In a nut shell your 545RFE has a .75/.67 overdrive... that compiled onto your 3.55 gears you end up with a 2.66/2.37 overall gear ratio... This means your engine will unlikely ever use that .67 overdrive and its most likely useing the .75 overdrive... which is still a 2.66 overall gear ratio this all equals BAD BAD BAD for anything heavier then a Pinto!

With 4.56's you end up with a much more practical and usable gear ratios... .75/.67 = 3.42/3.05 overall gear ratios... These are still considered GRANNY gears.

When choosing gears keep in mind that RPMs alone do NOT dictate fuel economy. So lower RPMs do NOT always mean better fuel economy as most think.

Its the load on the engine at ANY given RPM that has EVERYTHING to do with fuel economy. Chose a gear that will get you deeper into your torque curve and you will see improvements on fuel economy & overall performance.
_________________________________________

Here is some simple math to help you best decide on what gear ratio to chose...

_________________________________________

STEP 1:
Find overall gear ratio-
OVER DRIVE GEAR x REAR GEAR = OVERALL GEAR RATIO (OGR)

STEP 2:
FIND YOUR RPMS at ANY SPEED-
SPEED x OGR x 336 / TIRE HEIGHT = RPMs
_________________________________________

In my experience I have found that a good rule of thumb is to aim for 2100rpm's@60MPH. This gives us the best all round performance gains to include fuel economy.

If you have any additional questions please feel free to contact me.

SPEED SAFE, AIR RAM

Last edited by AIR RAM; 07-23-2009 at 01:13 AM.
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  #6  
Old 07-23-2009, 03:03 AM
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I was thinking that he had the 6-spd tranny that has a better overall gear ratio then the auto.......I agree that 4.56 gears will be way better for your application...
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Old 07-23-2009, 01:09 PM
euromade euromade is offline
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If I were to swap gears on my truck, what excatly do i need (it is a 4x4)? How much are these gears?
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Old 07-23-2009, 02:35 PM
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mlowney mlowney is offline
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gear swap Can't wait to see how this ends
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  #9  
Old 07-23-2009, 05:25 PM
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Quote:
Originally Posted by euromade View Post
If I were to swap gears on my truck, what excatly do i need (it is a 4x4)? How much are these gears?
You will need the kit for the front t-case and rear diff. There are a couple different gear manufacturers........Get a NEW Limited Slip Diff (Aftermarket) for your truck while doing this. Most shops will not charge anything more to install the LSD because they are already in there swapping gears. For the front and back you should come out + Gears around $1300.00. Sounds like allot but look at the $$ of new tires (that will wear out in 40k) and once you run that figure out you will see that getting the new gears is a better choice. This will make a 100% difference in your trucks power and possibly MPG in town and around the 55 mph roads. Highway mileage will be the same or lower because of RPM's (POSSIBLY)........ MAKE SURE that you find a mechanic that has been installing gears a long time..... Get references from your area or drive to a reputable installer. GET THE GEARS!!!!!!
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