Hey guys...I am going on Thursday to put an order in for a 6.7
I am curious...I tow a 4500 lb boat, with gas, etc...it probably equals about 6000 with people and supplies pulling out of the water.
Here is the quandry...3.31 for the gas mileage or 3.55 for the towing.
What is the difference in gas mileage on the two differentials?
And also, what is the difference in towing on the two?
Any help is appreciated!!

 

You will find that the axle ratios are somewhat dictated by the wheel size and he option packages. It can get confusing. But in the end, the 3.55's with a 20 inch tire are very close in RPM's per mile to a 3.31 ratio and 18 inch wheels. Someone posted the actual numbers here. Powerwise, no noticeable differences noted here.

 

I have 3.31's and they do fine towing, if you have to downshift to 5th going up a hill it will put you at about 2200 rpm doing 75mph which is perfect for me. these engines make a ton of power down really low, I say go 3.31's unless you want to run oversized tires.

 

I agree, this engine makes so much power and the transmission with its 6 gears will allow you to go 3.31 unless you decide to tow a big camper or something.

If I were going to use the truck without ever reaching the max gross weight rating I would probably suggest the 3.31 with 20" wheels, if you can get one.

 

I got the 3.55 EL with the 20"s. I think that's a good combination.

 

i average 16.5 mpg so far with 3.55s (diesel lariat crew fx4 4x4 3.55 ELD). would put a pic as my avatar and details of the truck in my signature if i could figure out how....but anyway, that's with limited towing and mostly around town/highway driving. the 2 ratios are awfully close together so i don't think there will be a significant difference between the 2 ratios as far as fuel economy anyway.....

 

once you have 25 posts, then in the usercp section (top blue bar, left)
you will be able to put a sig and avatar. FTE sets these restrictions for some reason.

sam

 

5000 miles............3.31 w/18" tires around town and some highway 17.5 mpg and all highway 20.5 mpg.

 

I think if I go with 3.55 with 20s I'll get very close to the same thing as cford

 

i'm pretty easy on mine so far (3.55 & 20s) but maybe it's not completely broken in yet. 1500 miles----> 16.5 mpg avg so maybe there is a difference

 

I've got 3.55 with 18" Continental Contitracs. Can't remember how I arrived at that combination, as the axle ratio automagically switched a couple of times while we were building the truck order with different options.

Anyway, with 8400 miles on it now (F350 6.7 SC SB FX4), I've had as high as 21 MPG on an 800 mile highway trip, staying under 70 MPH, an overall average of 18 MPG, and in the 15.5 to 16 MPG range for around-town driving.

Very happy overall, as those numbers are a good 10% higher than I was getting with my 2005 6.0, which was also 500 lbs lighter than the 2011.


Joe

 

You might want to investigate the replacement cost difference between 18" and 20" tires. That may be your deciding factor.

 

Ended up going with 3.55 with 20s and bridgestone ATs. On the drive home from the dealership today I got 18.1

 

I pull a fifth wheel Raptor toyhauler with a dry weight just under 12,000 lbs. My truck is a 2011 F250 6.7L CC FX4 with the 3.55 ratio and 20" wheels and recently flashed to 400 hp. Today I did 11.1 mpg over 70 miles of mostly level highway with the 5'er behind. I kept my speed just below 60 since I live on the "left" coast - Kalifornia where 55 mph is the limit with a trailer. Without the trailer and doing around 68 mph I get about 20+ mpg on the level road.

 

Just been reading this tread, 'cuase I had the same question. Want to say "THANKS" to all you guys who responded. Your information, suggestions and experience is really great. Wish I had this many folks to talk to when I bought my first F350 in 97.