Excuse my ignorance, but what is the difference between the 3.55 axle ratio and the 3.73 axle ration? Does the 3.55 ratio have a tighter turning radius and better handling? And what is "limited slip rear"? Thanks for any input.

 

Axel ratio is the number of revolutions the driveshaft make compared to the rear axle. In your examples the drive shaft would turn 3.55 or 3.73 times for each rev of the rear tires. Finally the lower the number the less engine revolutions needed for a given speed, better gas mileage. Higher the number better pulling power. That is why the HD come with 4.10.

Limited slip describes the differential. When you go around a turn the tires travel at different speeds because the outside tire has further to travel. The differential allows the tires to travel at the different speeds. In a open diff if you lose traction to the rear wheels only one wheel will spin. In a limited slip if you lose traction both wheels will spin.

 

Very, very simply - a higher number will give you more low end pulling power (better acceleration), and lower top speed, and, theoretically, less fuel mileage. Limited slip prevents one wheel from turning faster than the other (e.g. - spinning in mud or snow), causing the vehicle to lose traction and turn to one side.

 

0.18...........

 

Thanks, guys. To see if I get it. The 3.55 will give me better gas mileage and the limited slip will give me better traction in stuck situations. I guess that would be a good feature to have. Almost acts like a minny 4x4.

 

We do not order any 4x4 for stock without the limited-slip. It's our feeling that you really don't have true 4WD unless you also have a limited-slip diff.

 

Questions about Limited Slips - my friend at work recently bought a Chevy with the limited slip and complains it only works below 20mph or so. That above 20mph it acts like an open differential. Does the unit Ford uses have a similar feature?

 

You can't really place a speed on the feature unless the differential has some type of actuated locking mechanism. The new F150 does not have any such actuator. With a limited slip one rear tire can still spin independently of the other. The two rear tires are basically held together to spin at the same speed by clutches... these clutches with enough torque can slip allowing one tire to spin at a different speed of the other. That's why they're called "LIMITED" slip.

Some people don't like limited slip. For example... if you're driving along in 2WD and hit a patch of ice with a LS rear axle then both rear tires will break loose. If your rear tires are spinning then the rear end of the vehicle want to move to the front... causing a spin out... an accident.

If the same thing happens to a truck with an open diff. then only one tire will start to spin. The other tire will just roll. Believe it or not having one rolling tire is better than having two spinning tires. It is MUCH less likely for a truck with an open diff. to spin out in 2WD.

The same concept holds true when performing a burnout or peeling out. A truck with LS will break both rear tires loose and you'll have to slightly steer the wheel to keep the truck from doing a donut. A truck with an open diff. will only spin one tire and will just go straight.

If both of your rear tires are rotating faster than the front tires then the rear of the vehicle wants to spin around to the direction of travel.

I'm not saying LS is bad. I get all my trucks with LS. I'm just stating that it has it's disadvantages as well.

 

baja,

The reason the truck will spin out with a limited slip is with both tires spinning there is obviously no traction at the rear. With no traction the rear will go to the easiest place, usually down hill(crown of road) or to the outside if in a turn.

 

I bought a 2004 xlt with the 5.4 ,4x4. I occasionally go off-road ( dirt, snow or a little mud). I got the regular differential. Will my regular differential fill my needs or should I consider an after-market diff.?

 

this is my first truck with a limited slip rear diff, 04 fx4 scab 4x4 3.55LS. ive had two rangers prior (01, 03 4.10 rear end) among other older full size f150's without limited slip. i always did ok offroading or even on a sloppy gravel road. the thing to remember is that reguardless of what rear end or vehicle you have, the vehicle will only do so much. if you get in to deep beyond the vehicle capibilities, you will get stuck.

 

All good answers (above), but don't forget that the gearing is relative to the size tire your are using. As mentioned above, the 3.55 will give better gas mileage over the 3.73 for a given tire size range. Get above this range, say for 3.55 gears, and the performance will suffer, as well as putting additional strain on the tranny, gears, etc.
Didn't mean to complicate the issue, but you really can't talk gearing without mentioning tire size, and effecting the RPM's. The "higher" gearing number (ex. 3.73 vs. 3.55) will allow for larger tires that will come close to offsetting the RPM diff.
Use a gearing calculator - knowing the size tires you want to run and how you will use the truck (100% hwy, 50/50 hwy/off-road, heavy hauling, etc.) you can either pick the Gearing or Rpm and calculate the other (Rpm or Gearing).

 

In the case of one or both tires braking loose (as in hitting black ice) it would be the driver that easies off of the throttle to control the wheelspin. In all other traction situations.. snow, sand, mud, ect. with limited slip some of the traction would go to the wheel that has some. With the open differential one wheel is spinning, and digging a hole and you are calling a tow truck. Another advantage of a limited slip differential is that the spring loaded clutch pack pre loads the carrier and cuts down on slop in the rear end, saving sudden jerking loads on the gears under stressful situations.