A couple quick questions for you guys please. First off, did Ford offer the FX4 with a 3.55 LS, or was it just the 3.73 LS rear end?

If it were the 3.73LS only, would it be difficult to replace it with a 3.55 LS and what modifications would need to be made i.e. the ECU or PCM?

Thanks

Jeff

 

I have to ask...why would you want to switch from a 3.73 to a 3.55?

With the weight of your 4x4 truck, you will likely see no difference, or worse fuel mileage as the engine will have to work harder.

 

My dad had an 03 FX4 with the 3.55LS and enjoyed getting 20-21mpg on the highway. So he is stubborn and wants the 3.55LS

What kind of mpg do the newer trucks get with the 3.73?

 

MPG will vary with driving styles of course, but my 4x4 with 3.73's hasn't seen near 20-21 on the highway. How fast does your dad drive?? The newer trucks are heavier, and I see 16-17 at most on the highway in my 05 4x4 with 3.73's...but, I didn't buy a full size truck to get good gas mileage.

 

70-75mph. I get the same in my 2000 5.4 with the 3.55 rear.

 

You do know a gear change on a 4wd will involve BOTH axles and cost you about $1600 - that will buy a lot of gas.

 

To answer your first question Yes. From what I've gathered when you got the 20" wheeels on the fx4 package you automatically got the 3.55ls. Atleast thats how my 07 came and it has the tow package. The best I've seen to date is 19.1 mpg with a tonneau on my drive from Indiana to Maine and back. Its more like 13 driving around town and 14.5 if I actually get on the interstate for a good clip. And somewhere around 10 maybe 11 (on a good day) if I'm pullin my Javelin 379 FS.

 

I say 3.55 gears in an FX4 is kind of contradictory, but not only that, it's going to be costly to change just one small step in gears and minimal to no difference in gas mileage (<1 mpg).

 

You can accomplish the same thing by putting oversize tires on it.

 

Yes, but oversize tires take their own toll on gas mileage.

 

Before I lifted my truck, I got betwen 18 and 20 mpg highway between 65 and 70 mph with 5.4 and 3.73 LS. I also had custom tuning though. Most long trips I would average around 19.2 mpg.

 

It is all relative. The change from 3.73s to 3.55 is very similar to 33.5s to 35s. Neglecting the change in tire weight and rotating mass, the taller tire will net less mileage in town but could lower RPMs and possibly raise the MPG on the highway. Like I said, the change in gears is not too much, if highway mileage is sought it is really cheaper to buy a set of brand new 35s and rock the good looks and get the same higwhay mileage.