Ford 3.31 or 3.55?

It will make no difference in performance or fuel mileage of that truck IMO. The power available and the negligible difference in rear gear ratio coupled with the transmission gearing make it a non issue completely. If you had a 3 or 4 sp transmission it would matter, but you don't and you will never notice.

OpenRangePullen_Ford wrote:
A 3.31 doesn't belong in a truck, its useless. Get the 3.55 or bigger for sure, It will give you more useful power.

That's funny because not that long ago people like you said that 3.55 was no good in a truck and not long before that they were picking on 3.73. When diesel motors had 200 hp and less than 400 lb/ft, gearing made a bigger difference than it does now at 400/800 and beyond.

I have 3.55 with the 20" wheel option and towing 16,000 lbs is no problem at all. I have no problem maintaining greater than 62 mph in the mountains which is extraordinary compared to trucks just a few years ago. I was initially leery of 3.31 when they were first offered, but after 3.5 years of experience with the 3.55, I would not hesitate to get 3.31 with 18" wheels. If you intend to install larger tires at some point, I would stay with the 3.55, but 3.31 with stock 18" tires is going to net you the best fuel mileage and will still perform the same as a 3.55 truck with 20" wheels.

A 3.31 doesn't belong in a truck, its useless. Get the 3.55 or bigger for sure, It will give you more useful power.

Maybe some 6.7L owners can chime in with specifics but IMO there isn't enough of a spread between 3.31 & 3.55 to worry much about it.

In theory, the 3.31 will offer slightly better fuel economy while the 3.55 will provide slightly more power. Key word, slightly...7% difference between 3.31 and 3.55.

My 2 cents: gearing is more of a factor with gas engines than diesels. My 5.4L V8 in a heavy crew cab 4x4 3/4ton benefitted from changing to 4.30 gearing-- those extra RPM put the engine closer to its prime powerband.

Diesels make so much torque at lower engine speeds that gearing is less of a factor; not a non-factor, as deeper gears still bring a bit more torque to the table, but there does come a point where it's logical to ask if more is necessary.