Questioning my newly installed 4.56 gears

So far I'm having mixed feelings about my 4.56 gears...I drove to work today (80 miles one way) and used a whole 1/4 tank of gas. Granted I was running late and doing 75, and while I never came outta OD, my RPMs hovered around 2400 the whole time. Before the gear install I was getting 110 miles on average (hand calculated) with the stock 3.55 gears. Performance wise my truck felt awesome, and I know that I shouldn't expect to see phenomenal gains in MPG with lower gears (despite what others may say), but I didn't expect to lose almost 2 MPG. I know I'll probably see better fuel economy when I install my 6" fabtech lift and 315/70/17 BFG's, right now I'm just leveled and have 285/70/17 Nitto Terra Grapplers. But I'm having a little buyers remorse at the moment...was the loss of MPG because I was doing 75, or is this just one of the cons of running lower gears?

With 4.56 you will see better mpg in city worse on hwy compared to 3.55 if that makes sense.

going 75 has a lot to do with it. but in general 4.56's don't get along with highway miles. but in the city you should see an improvement.


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That is normal when running higher than 65mph or so on the highway. Your roms will come down some with the taller tires, but I never recommend 4.56's to anyone expecting to gain mileage. That is rarely the case.

Don't go 75-80

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the codes said:

going 75 has a lot to do with it. but in general 4.56's don't get along with highway miles. but in the city you should see an improvement.


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Hemi450hp said:

That is normal when running higher than 65mph or so on the highway. Your roms will come down some with the taller tires, but I never recommend 4.56's to anyone expecting to gain mileage. That is rarely the case.

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TheSatinPumpkin said:

Don't go 75-80

Swype-n from my Galaxy Note

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Lol, I figured that my speed had a lot to do with it. I don't expect to gain mileage when I add the lift kit and tires...just hoping to not have my truck be a complete dog and down shift on every hill. My end goal is to retain my 13-14 mpg average that I was getting before adding the gears, tires, and lift.

I've heard a lot of people recommend a SC 3865 as a future upgrade, what are the benefits of doing this first rather than a 206 h/o cam besides costs? I doubt I'll run a 91 octane tune, but it'd be nice to have that ability, and to remap my tranny.

On the Superchips, any of the tunes below the 91 tune are not going to show much of a performance gain. The transmission controls are nice, but if you want power, then you are going to want that 91 tune. Also, you might be forced to run the 91 tune after the cams.

I think you would be better off going through Hemifevertuning for a custom tune. He can help you dial in those cams and get the most out of your setup. He is a vendor here as well.

Hemi450hp said:

On the Superchips, any of the tunes below the 91 tune are not going to show much of a performance gain. The transmission controls are nice, but if you want power, then you are going to want that 91 tune. Also, you might be forced to run the 91 tune after the cams.

I think you would be better off going through Hemifevertuning for a custom tune. He can help you dial in those cams and get the most out of your setup. He is a vendor here as well.

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I'll look into the SCT...I need to find a cam first with an H/O grind. I figured finding an unlocked SC 3865 would be cheaper and easier. I could use it ro remap and play with it some while I saved up for a H/O cam.

Those tall gears won't make a lot of difference until you get up and over around 40 or 45 mph.

Sagebrush2500 said:

Those tall gears won't make a lot of difference until you get up and over around 40 or 45 mph.

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What makes you say that? I haven't really torn into them off the line, but passing at highway speed is a breeze now.

Sorry - I left out an important part of what I was talking about:

Those tall gears won't make a lot of difference IN MPG until you get up and over around 40 or 45 mpg

Once you put the bigger tires on it will make a huge difference on the highway. 4.56s and 33s don't get along very well on the highway. I did the same thing you did and felt the same way. Once I put the 35s on it was so much better.

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Update: drove 65 the whole way home I lost maybe two miles and never shifted outta OD. Only time will tell.

It also takes a good 500 miles for the gears to really break in. You will notice that the more you drive it thru better they will feel.

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I'm gonna break 500 miles in under a week probably. I'll have driven 320ish miles in two days

Any update on the gears?

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rhendrix said:

So far I'm having mixed feelings about my 4.56 gears...I drove to work today (80 miles one way) and used a whole 1/4 tank of gas. Granted I was running late and doing 75, and while I never came outta OD, my RPMs hovered around 2400 the whole time. Before the gear install I was getting 110 miles on average (hand calculated) with the stock 3.55 gears. Performance wise my truck felt awesome, and I know that I shouldn't expect to see phenomenal gains in MPG with lower gears (despite what others may say), but I didn't expect to lose almost 2 MPG. I know I'll probably see better fuel economy when I install my 6" fabtech lift and 315/70/17 BFG's, right now I'm just leveled and have 285/70/17 Nitto Terra Grapplers. But I'm having a little buyers remorse at the moment...was the loss of MPG because I was doing 75, or is this just one of the cons of running lower gears?

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A few notes from a fellow new 4.56 gear owner.

It's too late now, but you should have broken in those new gears correctly. Hopefully they did okay anyway. I checked my diff temp during break-in just by touching the cover by hand, and it was definitely hot, even following procedure. Change your diff oil ASAP, it's toast.

My truck formerly had 3.55 gears too and would generally get 13.5 MPG on a mix of highway and city driving. Highest it ever got on the hwy was 16 mpg one time. With my 4.56 gears and driving the same route to work, about a 17 mile RT that is 50% city and 50% highway, Im getting about 13 MPG now.

For me, I think if I turn MDS back on (it was awful with 3.55s) I will see an improvement and MDS will actually work. I also think I might see a mild improvement if I change the part throttle shift schedule to hit the upshifts 200 RPM earlier than currently.

Im currently running 285/70/17 Nitto Terra Grapplers too. I have 35s on another set of rims that I have not put on since I got the 4.56 gears. I think when I put those on, I will likely get the same gas mileage, because that’s what it did before with the 3.55s. If Im lucky the fuel economy may increase, but I will have to see about that.

Give us another report on how your truck is doing.

Best investment I've made. I averaged 16 mpg on the highway the other day driving to go hunting and never once came outta OD. The more I drive with them the better my truck drives. I haven't really dogged them except on a dirt road by my house. The truetrac kicked in as soon as I hammered it and I was flying. Still need to install the lift and tires, but its hunting season, those things can wait.

rhendrix said:

Best investment I've made. I averaged 16 mpg on the highway the other day driving to go hunting and never once came outta OD. The more I drive with them the better my truck drives. I haven't really dogged them except on a dirt road by my house. The truetrac kicked in as soon as I hammered it and I was flying. Still need to install the lift and tires, but its hunting season, those things can wait.

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Good to see everything turned out fine

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