what are the pros/cons of a 3.31 ratio regular axle in a 2010 Lariat SCREW 4x4? Im looking at a new truck that has it and Im a bit concerned. Should I be looking for one with limited-slip instead?

 

Since its 4x4 i wouldnt worry about not having a limited slip too much. Unless you dont like one wheel peels

 

I like a posi track rearend. It makes the 4wd drive a lot closer to actually 4wd. You have a 3 wheel peel instead of one front and one rear tire.

I have had them in the last 7 trucks I have had, and one Durango. I like the added traction in the snow.

ARe you going to pull with this truck? If you are, I would try and get a 3.55 or a 3.73. It will pull better.

 

Limited slip differentials are the best performance enhancing values you can buy. Turns one wheel drive into 2 wheel drive, or two wheel drive (part time 4WD, one front one rear with two open diffs) into three. For $250-300, you cannot do better. Since F-150's do not have automatic-4WD, then limited slip is still beneficial for every street situation. They are particularly useful for turning while accelerating, like pulling out into traffic from a cross street or business.

As for the ratio itself, I believe in buying the shortest one available (numerically higher). There is no measurable difference in real world fuel economy savings in F-150s.

 

If you plan on towing, I would not get it. 3.55 or even better, a 3.73 if towing. As far as other pros/cons. If you do a lot of city driving, your mileage will be worse, if a lot of highway driving mileage will be better than the others.
As far as LS, even on a 4wd, I like having the LS. Had one truck that was 4wd and open rear. Nowhere near the traction of a 4wd with LS.

 

To ALL, thanks to the feedback

 

Ford offers 3.15 to 3.73.
If going from a 3.55 to a 3.73 is about 200 rpm then going from 3.15 to 3.73 must be around 500 rpm and that would make a huge difference on fuel mileage.

 

I have the 3.31 open diff on my truck. I like it so far, highway gas mileage can be in the 20+ mpg range.

I haven't had any issue yet with an open diff. One advantage of a open diff is there are less moving parts to break. With the advancement of traction control is a LS diff needed as much?

 

That's a good question. I guess it all depends on how the TC works. Does it slow the slipping wheel, or both wheels simultaneously?

On the surface, a LS diff would still seem better, as there is a mechanical transfer of power to the wheel with more traction, no matter what (short of mechanical failure, of course).

 

I have the TC with the 3.73 locker. In all honesty, I would rather have a 3.73 LS. All the TC does for the most part is limit the power to the rear wheels. That and apply the brakes slightly on the slipping side.

 

I have the 3.73LS and love it. The difference in rpm's from a 3.31 to a 3.55 at 60mph is 100rpm's. The difference from a 3.55 to a 3.73 is another 100rpm's. So in a 4x4 with the stock 32'' tires at 60mph...

3.31= 1400rpm
3.55= 1500rpm
3.73= 1600rpm

 

I believe it closer to 150 Rs between the ratios. I probably would have given up the traction control for 3.31 if I was to do it over again. Unless you do lots of towing then I would stay with the lower rear ratio. I pull mostly a light snowmobile trailer and the newer 5.4 have the torque and hp to adequately pull my trailer.

 

I have the 3.15 with no limited slip.

 

18.4% difference in gearing between 3.73 and 3.15 axle ratios. So your 500rpm number difference come when the 3.15 axle ratio is at 2715rpm, which is 87mph in in top gear. Not a speed wear most people drive many miles.

Peak brake specific fuel consumption of an engine is not idle. Lower is not always better, except off. Lower engine speed does not equal less fuel used, at least at certain engine loads.