Now dont flame me, i did use search but didnt find my answer. I am a noob when it comes to gears. I just want to understand the difference between 3:55, 3:93's and 4:56's. Why does going to the 4:56 make a difference with a bigger tire. Other reason is i told a buddy about how you guys suggest 4:56's and he disagreed and well since i dont know i figured i would ask to be educated so i can do the same to him.

 

The 4.56's will alowe you to turn a bigger tire a lot easier and takes less strain off the engine. It does not hurt your MPG's, in fact in city mpg should go up and hwy will either go up a little or remain tha same. There are tons of guys running the 4.56's with bigger tires and smaller tires (have them in mine with a 31" tall tire and LOVE them). I'm sure more of them will pop in and explain a little more but dont listen to your friend...

DONT FEAR THE GEAR!!!!

 

It's the difference in rotational speed from the input to the output shaft. For instance, a common pickup gear ration is 3.55, which means that the input (drive) shaft turns just over 3-1/2 times for every time the wheels turn once. When you see numbers such as 3.55, it's actually 3.55:1, but the ":1" is dropped, since it's the same reference for all ratios. 3.55 gears are better for highway mileage, fuel economy, and creature comfort. 4.56 gears are better for acceleration owing to their increase of rear-wheel torque. Note that as the ratio number goes up, the trucks relative speed goes down, since the engine can only turn so fast.

 

To add to what Pacmanloads states, you have to look at the entire drivetrain from the output shaft of the engine to the tires. Since you're thinking about larger tires, I'll talk about those for a minute.

I like to look at a tire like a wrench, but the handle part of the wrench is the tread of the tire rather than the center. In this case, the axle shaft turning the tire is akin to the bolt turning the wrench, with the point where the road meets the tread acting as the hand that is resisting movement. Now, think about it for a minute - if you have a stuck bolt, you may try extending your wrench with a pipe or a longer handle (maybe a breaker bar). You're giving yourself a greater mechanical advantage with the longer lever. Likewise, a larger tire is like a longer wrench, except that it gives the road the mechanical advantage. So, you're trying to turn the tire to move the car but the larger tire has a greater advantage (or resistance to movement) over your force to twist the axle than the smaller tire does.

To overcome this advantage, you can change to a higher gear ratio. A 4.56:1 gear means that the input shaft and drive shaft turns 4.56 times for every complete revolution of the axle shaft and the tires. It also multiplies the applied torque in the driveshaft by 4.56. Compared to a 3.55:1 gearset, the 4.56 has a mechanical advantage.

Now if you increase your tire size, you're giving away some mechanical advantage, and it'll show in your acceleration and drivablity, especially when towing. To offset that effect, changing from a 3.55 to a 3.92, 4.10, or 4.56:1 axle ratio will do the trick.

 

It is the single most immediate effective mod. You install them, you break them in, you accelerate faster due to the mechanical advantage. Going up in a numerical ratio will lose you top end, but hey, it's a truck...bout as aerodynamic as a brick wall.

 

Frankly, I don't think these trucks can come anything close to topping out on RPM in the top two gears anyway. My guess is that the best top speed capability would come in 3rd gear if you're geared right. Your revs will be up in the power band, and you'll have the advantage of not being in overdrive. Not ideal for cruising, but for a test run, it'd be interesting to see what kind of top speed could be produced (on a track, of course - don't try this at home).

Of course, the governor is usually the limiting device.Any idea what the top speed governor setting is on these trucks? I know that it can be changed with a tuner, but I'm wondering what Dodge sets it at.

 

I think mine was 106-108. (Early 04 build 5.7L). Now i hit 120 at the top of 3rd (6100rpm) with 4.10's

 

Lot of factors at play here. First of all, as stated, the higher numerical gears give you a mechanical advantage. Secondly, you LOSE this advantage with bigger tires. You may have 3.92 gears, but going with say a 35" tire would make your EFFECTIVE gear ratio down around 3.55 gears.
Thirdly, with a 545rfe transmission, we basically have two overdrive gears, lowering rpms considerably at cruising speed. With a three speed tranny, you'd be taching 2800+ rpm at 65 on the hiway with 4.56s, so 4.10s or maybe even 3.92s would be necessary to maintain some semblance of fuel economy. But with our trannies, you would still only be taching around 2000 rpms at that speed which is right about at the beginning of the Hemi's sweet spot for best fuel economy/performance balance.

IMO, any tire at or over 33" and you don't need to be thinking anything else but 4.56 gearing...

 

I am also convinced thanks to the many great DF threads on this subject that 4:56 is what it will be for me cjs4444 and I hope to have that done by November!

 

My stock 20s with 32" tires weighed NOTHING compares to my current 24s with 36" tires. So Ive got more dead weight AND rotating mass. It takes more power to speed that up AND slow it down. So lower gears help speed it up and better pads help slow it down. Ive got a hydro boost I still have to put on though to help with braking.