Limited Slip Differential. To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)

The reason why you dont use four wheel low on pavement... Car wheels spin at different speeds, especially when turning. A buddy of mine owned an 01 F250. The rear end was a solid axle. The front had a solid axle. The front and rear of his F250 has leaf springs. The front of our F150's have independent suspension with a coil spring, not leaf like the rear. The right front tire can sit atop a curb, and the truck will be somewhat level because the tire on other side is not bound to react to the other. The suspension works independent of one another. The rear of our F150's has a solid axle. If one rear tire is on a curb, the axle is angled like that of a playground seesaw. One kid in the air, one kid on the ground. If an axle is locked, both wheels turn at the same speed. When you make a turn, pysics states that one wheel will make more revolutions than the other. Limited slip allows both rear wheels to turn at different speeds while still receiving power. I believe that in four wheel low, both rear wheels lock, and turn at the same speed. Does that help. Can anyone back me up, or correct me, or give better detail?

From a google search:
If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.

More Google:
Another time open differentials might get you into trouble is when you are driving off-road. If you have a four-wheel drive truck, or an SUV, with an open differential on both the front and the back, you could get stuck. Now, remember -- as we mentioned on the previous page, the open differential always applies the same torque to both wheels. If one of the front tires and one of the back tires comes off the ground, they will just spin helplessly in the air, and you won't be able to move at all.

The solution to these problems is the limited slip differential (LSD), sometimes called positraction. Limited slip differentials use various mechanisms to allow normal differential action when going around turns. When a wheel slips, they allow more torque to be transferred to the non-slipping wheel.

Did you ever see the movie "My Cousin Vinny?"

More Google:
If you've ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, because of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.

Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in.

Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn't take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn't going to move very much.

Hope I didnt just confuse more.

 

The reason we went through the boggy place was because we were coming out and it had rained - hard! We had to get back home.

I don't know what make or model vehicles the others were, the "getting stuck" had happened there the week before after heavy rains (the Arizona monsoon season) and the neighboring rancher pulled them out. Knowing what kind of vehicles most of our group drives, they were not 4x4 trucks. They could have been all-wheel drive cars though and they would have had very little ground clearance.

By the way, I've had a bit of experience in 4WD vehicles in all kinds of terrain. Much of it was in an Air Force jeep when I was a FAC (Forward Air Controller) out with the Army. I have a pretty good idea what 4WD can and can't do.

Wouldn't there be limited slip on both the front and rear?

- Jack

 

It has been many years since Ford has offered limited slips in both the front and rear. My 69 Bronco came with both from the factory...many years ago.

There are basically three different types of differential;
open
limited slip
automatic locker

Yes, there are spools, actuated lockers, etc. but that is beyond differential 101.

An open differential sends all the torque to the wheel with the least resistance. Great for a street only car, but not so great for a truck or off road vehicle. A good portion of vehicles sold have open differentials.

Limited slip differentials send torque to the wheel with the most traction to a point. They will never 'lock up' - sending a majority or all the torque to the wheel with the most traction. If it did people would start breaking things - bad for warranty.

Automatic locker will always send 100% of torque evenly to both wheels when power is applied. They will unlock when there is zero torque (foot off gas) and you are going around a corner. Great for off road, bad for street driving. This will give you strange handling characteristics and parts are very likely to be broken.

The above is very basic, back to factory offerings.
Having a rear limited slip will get you further in the mud, but there will be other concerns in play. Three major ones are ground clearance, tire tread pattern and speed. Any time you contact the terrain your truck will have more force to overcome. This is where the front axle pulling has an advantage, two axles are working to overcome the force-even if one axle has an open differential. Tires, a Super Swamper and a mud radial will both get you far into mud. A Super Swamper will get you further along in mud (that is what the tire was designed for). So either you'll be further in before you get stock or accross the bog. Speed is good for very short crossings in an underprepared vehicle or if your truck is prepared for mud/water. Speed gets a lot of people in trouble real quick, it will drown your engine out, mud/water will get into all crevices and it can suddenly change your direction of travel in a bog (hitting something unseen in the mud) causing loss of control or worse.

Your best course of action is stay in the tracks of the guy ahead of you and if there are ruts, do not travel the ruts if made by a truck with 35" tires and you have 33"'s. You will high center.

Experience is a good tool to judge your abilities by, not the truck brand - as many trucks have very similar or same drivetrains, just different brand names.

 

Mines been getting about 12.5 MPG since it was new :P

 

2007 F-150 KR 4x4 - my first tank was 10.1 mpg driving slow....took me around 4,000 miles before I saw vast improvement. I'm a little over 6,000 miles now and will get around 17 mpg on hwy but still around 13 mpg in town....

 

Thank you Corky Thats what I wanted to hear. 17 will make me very happy. Thankfully, my company pays me a car allowance and reimburses gas.
I previously owned a 00 F150. It had was paid off before gas his three bucks a gallon. With a new baby and single family income, I would have never bought my Kr Screww without the gas and car allowance. I feel so much better with 17mpg. There is no way I could justify 10 mpg for me and family. Some may not care, But if it only got 10...I think I might have returned it.

 

Yes, it will get better. During the first 1000 miles I wouldn't expect anymore than 10-11. I have a 2005 5.4, 3.73, 4X4. My mileage improved to 12.5 mixed city/highway after about 10K miles. At 29K I now get 13.5-13.7 in my mixed driving cycle and 17 on the highway at 65-70.

 

Good News! ! ! ! ! !

I drove from Frisco to Oklahoma City today and got 16.1 mpg. Since I was at 250 on the odometer, I drove at 65, 75, 80, 85, and a few miles at 90. I figure I average around 78, so I am very pleased with the results. It was a 450 mi. round trip. I am now at around 700 mi. I believe now, that the engine is more or less broken in, and can live with the gas mileage. Iwas pretty worried at first. Thank you guys for responding to this thread.

 

My 30K mile 2005 SuperCrew KR 5.4L 4x4 w/3.73s gets anywhere from 18-19 mpg on the highway. This is of course with the cruise control on at 68 mph (40 miles one way) and my shift starts at midnight. My stock block 261K mile (no joke) 93 5.0 GT w/3.73s gets 22 mpg if I keep it below 70 mph. I'm pretty happy with what I get. I hope to get moreout of the KR when I get an intake/tune/full exhaust.

 

I notice a big difference aka drop in mileage when I drive over 75 MPH. If I keep it at 70 or less I can get 18~19 from point to point on the highway. If I run 80 it drops to 15 real quick. These trucks have the aerodynamics of a brick.

 

YES! My gas mileage has been getting better as I put more miles on it. Just turned over 5,000 mileage and took it in for first service appointment. I was first getting about 13.5 mpg with mixed driving back in February, now I get about 15 with mixed driving. Lately, I've been getting about 16.5, but more highway than city. Mine is a 2007 KR with 3.73 gears and 18" wheels. I use low grade, which around here is 89 Octane.

I did a long distance trip a few weeks ago from Virginia Beach to Baltimore (okay, only about 4.5 hours). Digital dash fuel economy read 20.3 mpg. I calculated it out according to how much I filled the tank and distance travelled and got 19.5 mpg. It was all highway and had the cruise set to 70 mph. I have no tonneau cover and zero performance mods.

Very happy with my truck!