Rearend swap: 3:73 or 3:55?

The 3:70's in my T600 with 11R22.5 & 13sp were the perfect combo imo running the same area plus the midwest and SE. It would cruise and hold 70 mph with ease in hilly terrain. I could slow it down to and maintain 60 mph netting solid mpg's. It'll run 75 mph just fine all day too if that's what you want. You won't be downshifting all the time like these modern underpowered automatic slugs with the tall gear ratios.

 

Over the years of owning multiple trucks I have ran 3:36 3:42 3:58 3:73 3:90 and 4:11’s. My current truck is a 2015 international Pro star Cummins ISX with 14210b 10 speed and the very first thing I did when I bought this truck was to remove the garbage engine killing fuel guzzling 3:36 rear ends and replace with 3:90’s!

I agree with others of moving on up to 373s for your 13 speed, but it would be best with 3:90 especially with you being in the Appalachians.

 

I have 355's with an 18 speed, and at 70 with tall rubber I am around 1475rpms. So I would assume with 373's with 22 lopro you would be around 1600 rpms.

 

fuel shut off should be 2350, these are not lugging motors you will cause fretting on the mains

 

I would replace the transmission and go with a 13 speed would be my first option. but definitely with the 3.73 rear ends.