Help choosing gear ratio X15 605hp 18 speed

Awesome! Best of luck on your new rig. The setup below is with 3.25 rears, 22.5 rubber, 1500 RPM and the 18's .73 final drive ratio. You can go here to play around with the numbers and see what works best for you: Transmission Ratio RPM Calculator | Spicer Parts

 

Western Distributing in Denver specs all of their trucks with 3.25 gears. It’s like having 3.70 in 17th gear. I wouldn’t go 65 mph in 18th gear with 3.25 as you’ll be “lugging” that Cummins TRUST ME! Make sure you can run 70mph if you’re going with that setup. If I ever speced another Cummins powered largecar, I’d go 3.08 and run 17 th gear unless I were going >72mph.
I speced my 2014 389 Big Cummins with 2.93 gears....planning on running in 17th gear...it was still a tad to high hence the 3.08 idea.

 

I currently drive an X-15, 525/1850/13 speed (autoshift) with 3.36 on tall 22.5.

First- I freakin love this motor! It's been too long ago for me to compare it to the 6NZ's I used to have, but this thing is a beast. For what you're doing I think 3.25 or 3.36 would be fine. I'm California only & hazmat (55 MPH), running some decent hills and a fair amount of tight, roller coaster two lane. In my situation I kind of wish I had 3.55's, but this thing pulls so good it's hard to quibble. I do run a lot of 12th when I need to keep it right @ 55. On big hills I downshift about 1400, that seems to work well, although it will pull hard down to 1200.

I think you're gonna love it with whatever ratio you choose. This is the second X15 I've broken in and both of them gained power in weird little increments as they broke in right up to 100K. I love driving a truck that makes me smile when I see it every morning.

 

Got the tire height wrong. 22.5LP would be 40in. Tall 24.5 is only 44in

 

I've got a X15 565/2050, 18 speed with 3.25 rears on 22.5 lo pro.

Going uphill I'm in the hammer lane fully loaded. Thats about all I know...

And...

3.25 on 22.5 tires is essentially the same as 3.36 on 24.5 tires...

I bought my truck from Peterbilt Steve in Tulsa, OK, if you watch any of his videos you will see that the majority of his trucks are set up this exact way...... Its workin thats why.

 

I Only speak 18 speed ,3.70 rear and 11R24.5 rubber...... and the old timers used to think I was nuts because they liked 3.90-4.10 rears.......

guess I should get out more