I would have to agree with this... I still have the stock 3.92's with 35" tires and I can really tell that it takes more to move and for that matter stop my truck. I almost never drive on the highway, just in town, so my speeds never really exceed 55mph. In addition, when I start moving from a stop I spend anywhere from 10 to 15 seconds above 2000 rpm's and it is not uncommon to exceed the 3000rpm range (I have a bit of a lead foot). That might not sound like a very long time, but when you add up all the starts and stops; it is a fair amount of time spent in the mid-rpm range. The time spent in higher rpm ranges plus the larger tires has definatley reduced my miles per gallon. It is for these reasons that I want to do the 4.56 gears in the future. I have to believe that if I can get these tires moving with less effort and spend less time above 2000rpms that that will at least help my gas mileage. Now obviously I could make more of an effort to save gas by not stomping on it everytime I get the green light, but that is just not my style. I like to get up to the speed limit pretty fast.

 

ALL this talk about the 4.56 Gears... I have a few questions if anyone can answer!

I got a 2005 4x4 Hemi w/ the 5 Speed Auto. I have the regular 17" Rims, not sure on total tire size though. They are all factory/stock. I currently have the 3.93 or what ever gear that is (3.92 maybe?).

You guys talk about better gas milage going with a bigger gear (4.56). Now right now, I do mostly city type driving. Usually 40 and 45mph roads, some 55mph. If i'm on the express way, I usually cruise about 70mph. How much of a gas milage incrase can I expect getting the 4.56 gears doing city driving? And if I'm going 70mph for a long period, will my normal highway gas milage suffer? If so, how much? Or will it pretty much stay the same @ 70mph as it currently is with the 3.93 gears?

Also, forget who, and to lazy to go click the back button *Grin*... said if I already have the 3.93 gears, and upgrade to the 4.56 gears, I don't have to do anything to the front differential? How can this be? Or did I miss read that? Or does the gear/pinion set for the 4.56 gears come with two pairs; one for the front and one for the rear?

Because If you don't change the front, wouldn't the two be going different speeds/rotations when 4 wheel drive is engaged? Or am I just a complete moron? :P Be nice... hehe

Also, the wear and tear on the motor... obviously the engine will see less wear and tear due to the fact it never has to really go up in the RPM band (as much)? And less work required to turn the rear end *smile...*.. ?

I don't see any negetive about this upgrade. Almost scary. Obviously better take off, torque, less wear and tear on the engine, and better gas milage? Anything negtive really about this? Or? Would love to hear feed back! Really thinking of doing this upgrade.

My only other question, getting bigger tires or say bigger rims, will this be a negetive impact on gas milage and the rear end or positive? More gas milage? Less RPM turn out at the 70mph range? Let me know! Thanks.

-- K

 

I'm lost on going to lower gears for better mileage.. In my younger days of street racing we would put 410's and 456's for better traction and launches, not for better gas mileage.. Usually, a higher gear 290, 318, etc would provide higher gas mileage. What did I miss ?

 

Bingo. I'm probably no where as old as you *Grin*... but my mustang love... Bigger gears = lots of powa! Faster 1/4 mile times, lot less gas milage ... But I read a lot and everyone seems to agree 4.56 = better gas milage. How? I have absolutly no clue... I just think the turns it takes the drive shaft to complete a full rear axel spin is more... Which means more rpms to go the same speed. So I'm all confused?

I talked to my dad, he has a Hemi 5.7 RB and was confused. He said that can't be right! I told him to read and what not and he has spent some time reading and is really interested. He would love to take off quicker off the line and ++ get better gas milage! I guess its the over drive that really saves you the gas and the fact that the engine does a lot less work to get up to speed. So stop and go driving will save you gas milage with the 4.56 gears. I know this is only true (from reading) up to 65mph. Afterward, how much worse gas milage do you get? I do some highway driving and I will be doing more once i move down south from my work (WANT LAND!). So I will be cruising 70mph or so on country roads often. If it doesn't affect my 70mph at all, this is a for sure upgrade for me!

I think I sold my dad into it too. He is a bit confused though still. Thats why I asked my questions above your post. Waiting a response.

 

Hwy 4:56's = more gas used but in town where the speed might be 45-50 I can see where it will be less. Also usually larger tires go with the scenario and basically taller gears and larger tires = more of a stock type ratio. Plus like said before, you also have to take into affect the OD of the tranny. Now a 350 Camaro with a 350 transmission and no overdirve will eat you alive in gas because at most you are getting a 1:1 ratio. There are a lot of factors but the old days are not on us anymore. Remember the Dodge 5 sp tranny has 2 OD gears so that is what helps the fuel mileage with tallr gears. Now figure out a way to put it in 3rd and drive around a day like that and you'll be looking for some 2:XX gears!! LOL

 

Its all math guys... and math only has ONE right answer. SO dont take my word for it... although I have done the math.


You have a .67 over drive... if you want the overall gear ratio... meaning the ratio as if you had a 1:1 ratio like in a th400 NON overdrive trans....

transmission gear x rear gear = overall gear ratio

.67 x 3.55= 2.37

.67 x 3.92= 2.62

.67 x 4.10= 2.74

.67 x 4.56= 3.05

Now if you would like to see how much your RPMs change here is the formula:

SPEED x overall gear ratio x 33.6 / tire hight = RPM


You do the math on your own.... I have beat this into the ground so many times I already know the answer... Im driving it every day! But I would like to know when a 3.05 gear became too steep? As long as I could remeber a 3.05 gear has been refured to as a GRANY gear... and with 4.56 it is when in overdrive but when your not in overdrive you get the 4.56's you grew up fearing... AND THATS A GOOD THING .

Now consider the tire hights we have now days... you have much larger tires that where NEVER imagend back even in the early 90's... for as long as I could remeber the 15" rim was as big as you could get... and 26" tall tires where the tallest tire you could find... so ANY experience with 4.56's back when there was NO overdrive and short tires is worthless in this equation.... you need to take into consideration... vehicle weght, tire diameter, overdrive gear, and rear gear to make an informed decision on what is best... you can not just look at the rear gear and use old math.

SPEED SAFE, AIR RAM

 

ok my math sux cuz i couldnt get that to work AIR RAM but i think i was doing it wrong just got off work not to with it in the head right now haha. and i just wanna clear up when i said heavy *** 24 inch wheels i meant wheels my tires are 32.68 tall. so i think i am safe with the 4.10 for town and lot of highway and interstate. but then again my factory tires are only 30.5 (the small 17s) so i think might hurt a bit on the highway with them but i normally stay in town when i am on my 17s. now i just need to find the cheapest place to find a decent set of gears. thanks for the info so far.

 

My tires are only 29.75" tall.... and I feel the 4.56's are the ONLY way to go! So if your tires are taller your RPMs will be slightly lower than mine are....

Another fact guys... I dont have the .67 overdrive like most of you have... I have a .75 overdrive! So my RPMs will be slightly HIGHER than all of yours and I can still crank out over 20MPG with the shorter 29.75" tall tires.... This leads up to one of my first points... RPMs do not dictate fuel economey... the load on the engine has a greater effect on your MPG than RPMs.... Yes RPMs do come into play but teh 4.56s lessen the load of the engine at 65MPH where as a 3.55 gear would make the load greater.

There is NOT a hill on the highway that will make my truck downshift on... I can go up fairly steep inclines in overdrive without even downshifting out of gear.... thats mechanical advantage at its best.


Think about it! Most of you have 30+ tall tires and a .67 overdrive... so you will see BETTER results than I have seen... at least you will get the same if not better.

SPEED SAFE, AIR RAM

 

AIR RAM

You have the 4.7L, so your gas milage will be better then my Hemi. Will the Hemi engine see the same thing though? As far as milage increase and such? And what where you getting around town and what do you get now with the 4.56s? And what about Highway? I'm just curious what kind of increase I can expect. And if I do go say 75mph for a long period of time, will my gas milage suffer? Will it drop below what the 3.97 will give?

Also, was your a 4x4? I'm still wondering if I have to replace the gear in the front differential? I am extremely interested in this mod :P

 

What do ya think it would cost to have the dealer do it? This way if anything goes wrong its on them. I cant see it being more then 2g's