I am planning to purchase a 2018 f150 with the 5.0 and 10 speed transmission. For best overall fuel economy and some light towing should I go with the 3:31 or opt for the 3:55.

 

3.55s without hesitation. I would be incredibly interested in someone proving to me that 3.31s actually get better mileage than 3.55s in real world driving situations over say a 20k mile test period.

3.31s just arent enough gear for combating any sort of rolling resistance at highway speeds in overdrive. And if you ever added any extra weight from a heavier tire or whatever, now its even worse.

 

I have the 3.31s and they're just fine with towing and normal driving

 

I've been extremely happy with the 5.0, and 3.31 gears. Here is the latest trip results, that includes a trip to the NC mountains last weekend:

 

In 2017, in order to get the 3.55 gears, you had to order the MAX tow package (another $1300). Anyone know if this is true for 2018?

 

I don't think it is true for 2017. Not sure about 2018 but would be shocked if it were.

I also have the 5.0 / 3.31 combo. If I had to do it over again, I would not buy a 5.0 without 3.73s. That said, a typical highway trip (Charlotte to Atlanta in this case) running about 70 mph yields around 20 mpgs for me.

 

I have the 3.31 and I do fine thus far. However, if I could I would have gotten the 3.55 just for more flexibility when it comes to towing and running bigger tires. I think the fuel savings is pretty minimal with the 3.31. If you have a choice go 3.55.

 

Maybe you're right, that'd be nice. But from Ford's "build to own" site:

 

The 3.31s will get you slightly better fuel economy. I have them and I think they're fine, even when towing my 2 ATVs. I have to downshift to maintain speed up some hills, but that's why there's 6 gears in the transmission, not just 1.

 

in the '17+ with the 3.5EB and the 10sp auto....the difference between 3.31 and 3.55 is minimal.