I have a 2001 4x4 (67K miles) 5-speed with 3:55 open diff. on 33's, and fresh Royal Purple front and rear. I am considering Line-X'ing my whole truck or getting 4:10 limited slip gears. I wanted to run OEM parts because I get them near cost, but my dealer says they only list the 3:73's. What are you guys running? I want factory or better reliability, but don't want to pay an arm and a leg to replace a perfectly "good" drivetrain. Also, would you recommend a small shop or dealership for this job? They tell me I have to drop the whole front axle and that alone almost scares me off since I've just replaced a lot of the front end components and don't trust most people's work. Then there's the install itself and making sure it's running / wearing properly. I read you are suppose to drop the fluid and change it again at 500 mph too. Is it really worth it? Since I've never done gears, what kind of difference will I notice? Does the truck feel lighter or does the motor feel stonger? If I had a 2WD I wouldn't think twice about it, but 4x4 makes it a PITA. How much can I expect to spend start to finish?

 

Don't waste money on 3.73 gears. Get from the aftermarket if you have to.

JMC

 

Finally found a local gear shop. They suggested for my exact app., that I run 4.56's. What are your guys's thoughts on this? Seems like if I had 35's + that would be the way to go, but 33's??? I'm looking at $300 in parts for the rear, $300 parts front, $300 labor rear, and $400 - $550 labor on the front. This is in my price range, but if I add a "locker"...tack on another $500. Guess there is a downside to 4x4 afterall.

 

Not sure if thi helps you or not...
I run 4.10s on street tires, effectively about 31 inches (285-60R18).
I think they are well suited to my setup.

4.56 isn't too much shorter than 4.10...and your 33s are a little taller than a my 31s. I don't think 4.56 is too short, but that's just my opinion, because I also tend to think that these heavy trucks really love shorter gears.

I would change my opinion and say 4.10s if you did a significant amount of 80+mph highway driving.

But I don't think you can go wrong with either one for mized driving. And if you do mostly city, or tow, then I'd say 4.56.

Let's see if any other chime in...my guess is that most opinions will be for 4.10s.

 

Hey man, didn't see this until now...thanks for the reply. I mostly drive highway, but 35's aren't an impossibility in the future either so 4.56 may be a good option.

Considering how "in tune" I am with my truck, I'd imagine regearing this much, especially on a 5-speed will really make a difference. I've never done gears so I have no idea what to expect. Can anyone really put into words what it'll feel like? With pulleys, e-fans, and a port matched upper intake with SCT tuning, it should be pretty snappy...???

What RPM range is desireable at highway speeds in these trucks? I have run 70 in 4th. gear since this post and IIRC, I'm still only turning about 2,600 RPM. Any idea what approx. "gear ratio" 4th. gear ends up being in 4th. on an M50D with 3:55's?

Can anyone confirm whether or not I'll have to drop the front axle for this job? Are lockers really worth $500 more? Will my mpg's improve? Are the take-off parts worth anything? TIA!!!

 

Here are my RPM at various highway speeds:
https://www.f150online.com/forums/sh...d.php?t=263323

I've obviously got an automatic in my Expy.

Lower gears multiply the torque that get to the wheels.
So the perfomrance improvement will be more significant than a typical bolt-on modification, IMHO, because you are creating a superior mechanical advantage.

Personally, my opinion is that the peformance gain of 4.56s will be superior than all of your current mods added together.

It's like changing gears on your 10-speed bike when pedaling up a hill.

The two best mods I've ever done:
1. Procharger
2. Gears

 

Thanks again buddy...we'll see what the tax return brings. I may do this even if I can't afford a limited slip. 67K miles and I haven't REALLY needed it yet so I suppose I could live without it. Think 4.56 is the way to go though for sure. Did you swap from 3.08, 3.55, or 3.73?

 

I had 3.31 from the factory.
It was a total dog.

I put LS in when I did the swap. I figured they were already in there, may as well save a few bucks on labor and have it all done at once.

 

At 70 mph you should be turning about 2025 rpm in 5th with 3.55 gears and your Procomp ATs. If you go to 4.56 gears you will be turning around 2600 rpm.
If you get 35 you will lower the rpm to about 2450. With those gears you will find 1st gear very short.

JMC

 

Thanks for the conversion there JMC, are you saying 4.56's are too "short"?

 

What I am saying is that 1st gear will wind out in a flash. I have 33 MTs on mine and I am through 1s gear in no time. I only have 3.55 gears. If I were to change I would prefer 4.30:1 but I cannot find any. If you are planning on changing to a 35 inch tire I would not hesitate to run 4.56:1 gears. The extra weight of the 35 inch tire over stock is a tremendous increase in reciprocating mass. If you are going to stay with the 33s then a 4:10 would be enough to over come the extra weight and still give you fair highway mileage. The overdrive on the 5 speed is only 0.80:1 vs the auto that is 0.70:1. That would mean 2275 rpm @ 70 mph with 33s and 2145 rpm with 35s with the automatic. That is a big difference in overdrive numbers between the 5 speed and the automatic.

JMC

 

Right...just wondered if you felt that was too "short". I just want to make sure I do enough to feel like I got my money's worth. Also, these ProComps weigh 60 lbs. a piece on top of whatever my 17x9's weigh.

For now, I plan on staying with the 33's, (32.6") and these will likely last me 4 years with the little driving I do now.

...so assuming the #'s you gave me in your first reply are for 4.56's with the 5-speed, and in your last reply are for 4.10's with the auto tranny...would the 4.10 RPM's in a 5-speed be slightly higher or lower than for the auto?

I really appreciate the help guys!!!

 

x2
I think that sounds like good advice.

 

Does anyone know if they'll have to drop the front axle? Even if they don't...does this job require removal of the hubs or anything more than just the wheels front or rear? I don't like the idea of tearing everything apart unless I'm doing it myself. I think it's begging for problems, and why pay for all that unnecessary labor? Too bad you can't achive the same results through the tranny. I wouldn't mind so much if it wasn't a 4x4...I'd have done the rear long ago.

Suppose I could spend the money to have the "seeping" driver's side headgasket replaced instead.

 

Ill be going to 4.10 ls from my 3.55 open eventually. Just on street tires.

Oh and FYI ford racing has a 4.10 gear