6.4, 35s, and 4.56 MPG

[FlatbedHemi]

OK, I know what you are thinking. "Why do you buy a 3/4 ton truck for mileage?" "If you want mileage, buy a Prius." "Blah, blah, blah." And, yes, you are right. I didnt buy it for mileage, I dont want to buy a diesel, and I am not going back to stock. Anyway, I think that about covers the basic smart-A responses to the question I am about to ask.

Also, I have scoured the Ram forums looking for an answer, but everything is either being reported as mileage while towing, pertaining to 4.10 gears, or for 37" tires. Cant find my exact setup.

So, I have this 2500 pickup with a 6.4 Hemi, 35" tires, a skirted flatbed, and factory 3.73 gears. I love the truck more than any one I have had to date, and I have had a lot. The only complaint I have is that when I put the 35s on it, while it looks awesome, it has a bit of a lug in 6th gear at speeds between 60 and 70, which is where I drive most of the time. The mileage I can live with, but I would like to get a little more "umph" to it to be able to hold 70 a little easier in 6th gear. So, I have considered having it regeared to 4.56 gears after I pay it off. My thoughts are that if I put lower gears in it, it should be able to handle itself better on the highway and allow the MDS to kick in at highway speeds, which at the very least might help offset the higher RPMs that I will be turning. Plus, the added capability when towing would be a big plus.

While I am OK if the mileage stays the same, I dont want it to get any worse. Anyone have any personal experience with the 6.4 Hemi matched to 4.56 gears that can report on before and after mileage?

 

[RLJ10X]

I hate when a gas engine lugs. Your truck will be a lot happier with shorter gears.

I would definitely regear. My only question would be wether to do 4.10s, or 4.56.

Your gas mileage won’t be effected much. You’ll be a lot more satisfied with your truck.

 

[FlatbedHemi]

I hate when a gas engine lugs. Your truck will be a lot happier with shorter gears.

I would definitely regear. My only question would be wether to do 4.10s, or 4.56.

Your gas mileage won’t be effected much. You’ll be a lot more satisfied with your truck.

I would go with 4.10s, but Im not sure if that will be worth it for the cost of a swap. I had a half ton Chevy with a set of 4.10s in it, and I loved it. It had 33s but it was only a 4 speed transmission with a .7 overdrive, and a much lighter truck. Im pretty sure I want to go 4.56 at least. I have gotten reports that people's mileage has gone up with as low as 4.88s, but I dont think I want to go that low.

 

[RLJ10X]

I get your point. Which ever ratio you choose, you’ll be a lot happier with your truck.

4.10s, in my application is just right!

 

[HammerHead]

Not much different between 3.73 and 410. I’d go 488 Minimum.

 

[RLJ10X]

4.88s sound like a little too much of a good thing. But I have no first hand experience with 4.88s in that application. I'd find someone with 4.56s, and 35" tires and ask them what the rpm in top gear is @ whatever speed is your highway cruising speed.

I think my engine is spinning right at 2000 @ 70, which I like.

 

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I have 4.56's but I'm running 37's. Love them probably could have went with 4.88's. Getting around 11 to 12 MPG running premium gas and between 70 and 80 MPH around 2000 RPM.

 

[FlatbedHemi]

View attachment 162575 4.88s sound like a little too much of a good thing. But I have no first hand experience with 4.88s in that application. I'd find someone with 4.56s, and 35" tires and ask them what the rpm in top gear is @ whatever speed is your highway cruising speed.

I think my engine is spinning right at 2000 @ 70, which I like.

DANG! You only have 43K miles on a 2011. Thats nearly brand new! LOL!

Thats about where 4.56s would put my truck with the 66RFE transmission and the 35s. Thats kinda where Im shooting for. 4.88s would be like running my truck in 5th gear, and it just seems a little too low for me, but I cannot attest to that for certain because when you kick it into 5th, it turns off MDS anyway. Ideally, I would like to run both setups for about a week or two to see what the differences are, but thats not anywhere near realistic.

You guys running the 4.88s, how does your truck hold the MDS mode? Thats the main reason for doing the swap for me. I just want it to hold eco mode when cruising and not downshift when going through the slight rolling hills we have here in East Tx.

 

[LeesEvoX]

DANG! You only have 43K miles on a 2011. Thats nearly brand new! LOL!

Thats about where 4.56s would put my truck with the 66RFE transmission and the 35s. Thats kinda where Im shooting for. 4.88s would be like running my truck in 5th gear, and it just seems a little too low for me, but I cannot attest to that for certain because when you kick it into 5th, it turns off MDS anyway. Ideally, I would like to run both setups for about a week or two to see what the differences are, but thats not anywhere near realistic.

You guys running the 4.88s, how does your truck hold the MDS mode? Thats the main reason for doing the swap for me. I just want it to hold eco mode when cruising and not downshift when going through the slight rolling hills we have here in East Tx.

Just fyi. If you use the manual gear selector it shuts off MDS.

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[FlatbedHemi]

Just fyi. If you use the manual gear selector it shuts off MDS.

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I realize that. That’s why I said it in my original statement. Lol. My end goal is to allow the MDS to do its job while running 70-75.

 

[huntergreen]

Did you have the computer set to recognize the larger tires ? If not your speedometer and shift points are off.

 

[FlatbedHemi]

Did you have the computer set to recognize the larger tires ? If not your speedometer and shift points are off.

Have a Hypertech supposed to be here monday.

 

[huntergreen]

Can hypertech change the tire size on the pcm ?

 

[LeesEvoX]

I realize that. That’s why I said it in my original statement. Lol. My end goal is to allow the MDS to do its job while running 70-75.

Oh right.. dur moment. Sorry bout that lol

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[Ratket]

This topic is heavily covered in the HD section- you would have more responses inquiring in that section or on those threads.

With 35”s I would go 4:88”s.

A switch from 373”s to 4:10”s is absolutely worthless.

 

[FlatbedHemi]

I have been searching that area, but very few people that I have seen are running the setup I am asking about, and not many give feedback as to their fuel mileage. Dont know if I am missing it or not, which is possible since Im new to the forum.

Im slightly concerned that 4.88s might be too low for my driving situation. I know they will net me about the same, maybe slightly better mileage, but I am wondering if the 4.56 would be better or worse for mileage. I know anything is better than the 3.73, but I am trying to get the best balance of power to MPG.

 

[crackerjack1957]

If your top priority is fuel mileage the 35" tires probably cost you at least 2 mpg.
4.56 will give you better mileage than 4.88 but less performance.
Always a trade off, can't have the best of both worlds in a heavy truck.