Gear ratio swap for highway

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Meatpuppet

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Hello everyone.
I have a 2003 2500 Cummins with power upgrades with a nv5600 6-speed, AAM 11.5 373 gears. I use it to pull a 5th wheel RV. The 1st gear I use for maybe 3sec and 6th gear revs a bit high cruising down the highway, feels like I could use another gear. I think the best way to improve the truck is to change the ratio to 342. What is the best option moving forward change the ring/pinion or can I swap out a used diff from a newer ram with a 342 already in there?

Thanks
 

OC455

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When you said it revs high, what RPM are you running?
 

06 Dodge

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My self I would go to bigger tires than spend the $$$ for 3.42 gear change over, most of those who have 3.42 with auto trans hate the 3.42 gears when it comes to towing. 2200 rpm at 110km is 68 MPH, do you think those RPMs are to high or will hurt the engine? That engine with good oil should be able to run 2200-2400 all day long without problems....
 
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Meatpuppet

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Was thinking 1900rpm would be better on fuel. I have 265/70/17 on the truck now. Was thinking of going bigger tire but need to have the 5th wheel trailer level. perhaps 285 would be ok and not have the trailers nose to high.
 

MikeT

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My 06 with the RFE545 auto trans runs about the same rpms in tow/haul mode and about 1950 when not towing. I think you are right in the ball park.

Mike
 

chopperman1

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Keep in mind, that if it's a 4wd you would need to change the front as well. How many miles to you think it would take to re-coup the 2k+ cost to gain maybe 1 (maybe2) mpg? Even if 2wd, same thing, 1k+ for 1mpg. Simply not worth it for fuel economy. Tires, maybe, but only if you need them, again not worth 1k+ for 1mpg.
 

Ole Buck

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Keep what you have. You will hate yourself for getting a higher ratio. You will not like it.
 
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Meatpuppet

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Very good points, thanks guys! Will probably buy slightly bigger tires next time I need them.
 

06 Dodge

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Very good points, thanks guys! Will probably buy slightly bigger tires next time I need them.
Just remember when you go to bigger tires your speedo will be off you will be going a bit faster then what it reads... I recall in my 06 with 5.9 CTD got its best fuel mileage at 55 MPH but still did well up to 2100 rpm's...
 
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Meatpuppet

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I think the speed info is taken from the wheel sensor so should not affect the speedometer.
 

crash68

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I think the speed info is taken from the wheel sensor so should not affect the speedometer.
Changing tires will effect the speedometer. It's when you change gear ratio that the wheel speed sensors are not effected.
 

Zoe Saldana

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Was thinking 1900rpm would be better on fuel. I have 265/70/17 on the truck now. Was thinking of going bigger tire but need to have the 5th wheel trailer level. perhaps 285 would be ok and not have the trailers nose to high.

Do the math.

You will never pay for the swap with fuel savings.
 

Blue Truck

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I have your exact setup, and I would NOT change the gearing. I don't use 1st/granny unless I'm towing or need to creep slowly. Currently running 285-70/17's and will soon be running 255-80/17's. That's 1.1" and 1.5" taller than your tires. Granted, I have a Gear Vendors overdrive unit, which I only use when I can hang at around 70MPH. Oddly, the GV doesn't seem to improve my MPG. Aerodynamics... Haven't corrected the speedometer, as the 4'ish MPH it's off at 60MPH doesn't bother me.

Daily driving, the 3.73's are close to perfect, except for the higher revs at 70MPH. I think I'd hate the 3.42's for towing. You mention having a 5th wheel, so unless it's a featherweight... If I didn't have the GV unit, I certainly wouldn't buy one; it came with the truck.

Unless you're in the prairies and have lots of high speed driving, I'd highly recommend you stick with the 3.73 gears.
 
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Meatpuppet

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Ya I will probably try the 285/70/17 next time but will look into the 255/80/17 ( Is this the same as 33inch? ) thanks for all the info very helpful!
 

Blue Truck

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Ya I will probably try the 285/70/17 next time but will look into the 255/80/17 ( Is this the same as 33inch? ) thanks for all the info very helpful!
Here's a great site I always recommend for comparing tires. You can use the calculators on the left side to figure out sizes of different tires or compare two different tires for height, width, etc. One of the tabs gives details about a particular size, and the other allows comparisons between two tires. The "Tyre Calculator" one even shows pricing (U.S.) of different tires and has other helpful links.

 
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