Ok, so I ordered a 2011 FX2 with EcoBoost and the 3.15 rear end. I drive about a 100 miles a day and prefer good fuel miledge. I'm concerned I should have ordered a 3.55. Sounds like I can amend my order and change to the 3.55.

 

If you plan on hauling or towing more than a box of froot loops I'd see about changing. Also depends what speed you'll be going. I find even my 3.31s not any more efficient at speeds lower than 65mph. My truck shifts on most hills I encounter cause I'm only running 1600 at highway speed.

 

I drive to work daily in Dallas traffic. In this traffic I drive stop in go about half the drive. The other half is 60-70 miles an hour. I'm currently driving a 2009 XLT 4.6 that will register 19-20 miles to the gallon with a 3.31 rear axle. I hope to get a little better fuel economy but would enjoy a great 0-60. Thanks for your input.

 

Search for your topic, there are several opinions out there that believe gas mileage may be lower with a 3.15 instead of higher because of more downshifting to lower gears when encountering hills and also amount of stop and go traffic. You have to decided what your driving envirnoments will be. Again, do some searches and see if what they say may apply to your situation.

 

I think the most important part of the question - How fast do you plan on driving it? The 3.73 makes a lot more sense for city driving and states with 55 MPH speed limits, and the 3.15 makes more sense in states with 75 MPH speed limits - assuming you will be on the interstate most of the time.

Here's some examples - with 31.5" tires (I believe that's the stock tire size) - I used 60 MPH cause I'm lazy.

At 55 MPH, I'm almost sure the 3.15 would have to downshift to climb even the smallest hill, while at 75 it should spin up the turbos and pull.

3.73 rear end @ 60 MPH in 6th gear - 1,451 RPM
3.15 rear end @ 60 MPH in 6th gear - 1,225 RPM

3.73 @ 60 MPH in 5th gear - 2,046 RPM
3.15 @ 60 MPH in 5th gear - 1,728 RPM

5th gear on these trucks is 0.86 overdrive, 6th gear is 0.61 (for ecoboost).

At 71 MPH, the 3.15 rear end will be spinning at the same speed as the 3.73 rear end was spinning at 60 MPH.

The torque curve from 1000-1700 RPM is very steep - at 1500 RPM it looks like you'd get 350 lb-ft of torque, and at 1250 RPM you'd get about 320 lb-ft.

 

I'm considering changing from a 3.15 to the 3.55. Seems. Like it would be better low end but wont give up to much on the high end.

 

Ok, I changed to the 3.55.

3.73 sounded great but I prefer the better gas mileage.

 

Of course if you ever plan on putting bigger tires on it and get the 3.73, it will be closer to 3.55 then.

 

Have the 3.55 EcoBoost, you will not be disappointed with the get up and go. Have not towed yet. Looking forward to that. I think you will be much happier with the 3.55

 

I can't believe people analyze fuel economy so much when buying a full sized truck