In all of the research I am doing, I ran across two fairly similar KR trucks. One has the 3.31 axle, the other has the 3.55.

Obviously (it appears) that running lower RPM's with the 3.31 may give a bit of a bump in fuel mileage. The 3.55 should be a but quicker and more towing capacity.

Has anyone driven both and have real world thoughts between the two?

Thanks!

 

I just drove my truck for the first time. I have the 3.31. Was doing around 1,400 RPM at 60 MPH and 1,6000 RPM at 70. I barely have 300 miles on it but so far according to the display I am averaging 11.6 l/100km - 20.2MPG US in mixed driving. At 60MPH I was doing better than that and a 70 a bit worse. Definitely better mileage than my 2013 F-150 with the 5.0l. I have the plus sized 18" tires so that keeps the RPM's low. When towing I will lock out 6th which should effectively give the same ratio as the 3.55. I did drive a 2016 F-350 with the 6.7l and a 3.55. Too long ago to tell which one was quicker. I suspect the bump in hp with the 2017 would make the 3.31 almost as fast.

 

Did the one with the 3.55 also have 20 inch wheels? If so then the final drive ratio will be almost identical due to the larger tire. The only real difference would be a 3.55 with 18 inch wheels. Even then not sure you'd notice a big difference. If i recall correctly the max tow ratings are nearly the same. I ordered the 20 inch rims because I liked the look better.

 

I think you will not need to lock out 6th when towing my friend had a 2011 6.7 cclb with 3.31 it towed his 38ft 5th wheel just fine he never noticed a lack of power it would down shift on a hill but would stay in 6th all day on the flats. Only reason he got rid of it was to get a dually for better sway control

 

I have had 3.31 and 3.55. Never really noticed any fuel mileage difference. But I believe like someone said to order a KR with 3.55 you also need to order the 20" wheel and tire combo. At least that is what I was told at my dealership.

 

If you order the 18 inch wheels then you can order either a 3.31 or a 3.55 rear axle ratio. If you order the 20 inch wheels then the 3.55 is required.

 

Yes I wanted the 3.55 ratio but the 18" wheels on the KR that |I wanted to order but was told no. But you do know it was the salesman that told me that. Ordered a Platinum instead.

 

He might be right, but I'm always skeptical of salesman. It's been my experience that I usually know more about the truck then they do. I had one tell me I HAVE to have a dually because I have a fifth wheel trailer, without knowing anything about the size of the trailer. Anyway, if you build one on line it does allow choosing a 3.55 locker with 18 inch wheels. Maybe the website is wrong.

 

Yes I said that about the online thing to the salesman and he tried on his system and it would not let him build it that way.

 

Huh that's interesting cause it also shows the 3.55 as an option on the XLT and you can't even get 20's on an XLT. Oh well I guess I stand corrected! I never actually tried it as I wanted the 20's with a 3.55.

 

I have the 3.55's in my truck now and wished the 3:73's was still an option as I always go to a larger Diameter tire than stock for off road clearance plus better traction than the stock tires...


Sure the 6.7 has more Tq than the previous engines and I'm happy with it but would've still chosen the 3:73's for mine.

 

It could also be a Canadian thing. I have a nice set of 20" wheels with 35" tires that I wanted to put on the truck. That is why I wanted the 3.55 and not pay for the 20 wheels.


Back to the OP question and 3.31 and 3.55 it did notice a power loss when I replaced my 18" wheels (3.31 gears) with 20" wheels and 35" tall tires (kept the 3.31 gears).

 

Thanks for the comments everyone! I think I will stick with the 3.55's (when I find the right truck), in case I want to go to 35's at a later date.

 

I ordered my KR w/ 3.55 (locking) and 18's no problem. I did not get chrome though, just caribou wheels.