Need axle ratio advice/New 2018 389 505hp X15 18 spd 3.21 ??

I've got 3.25 in a 579 with a 13 speed. Near 80k loaded all the time. It pulls fine. And the mx13 is doing well at 400,000 miles. I get 7mpg to 8mpg. The isx I doubt will get better than 6.5.

 

Should pull good, 3.08s used to be “west coast gears” 3.55s in mine does good on pull while still gets decent mpg.
The 18 speed should let you run as if empty with the gross weight you say

 

I wouldn't get any gears slower than that
3.21 even 3.08 will work great for that.

I'm using 500/1850 isx at 80k with 3.73 and 11r22.5, this truck is made to pull a lot heavier weight than it does. I would be happier with 3.55 or 3.36. But for now it gets the job done good enough.
Edit
I get high 5s low 6s summer, winter 5.2-5.5, no idling, always heavy, mountains and prairie wind kill it in winter especially. However, it overall averages about .5 mpg worse than my previous s60 classic. Which I attribute mostly to dpf.

 

A friend of mine is driving a 2018 389 with the X15 Efficiency 505/1850 and gets 8-9 mpg loaded to Canadian weights (100,000 lbs gross) in the prairies pulling deck. It's been in the shop a lot for SCR/DEF issues, but since the turbo speed sensor got replaced, there hasn't been any problems at all. He's over 60,000 miles now.

When they work, they are fantastic motors. The 565/2050 Performance motors are apparently the "perfect tune" for them.

Anyways, @Rowdy Finn , if you aren't pulling around more than 50,000 gross, you'll have more power than you'll know what to do with. The 3.21 rear end might slow you down a bit on some bigger hills (actual grades) but you likely won't be dropping more than a half step or two.

 

What is the engine's recommended operating range? At what RPM is it most efficient?

What is the top gear ratio in the transmission?

What size tires?

How fast will you typically set the cruise to run?


Pick the gear ratio that puts the engine at it's sweet spot...where it operates most efficiently...at the speed you intend to run. If you're going to run 65, it'll be a different gear ratio than if you're going to run 75 all of the time. If you spec it for 75 and run 65, you won't get the mpg's as you would've had you spec'd it for 65...and likewise if you spec for 65 and run 75, your mpg's will suffer.

 

I would spec it for MPG say 60-65 MPH then run the transmission in Direct Drive(the most fuel efficient gear). Then pick the rear end to match that 60-65 MPH road speed.

If the 18 speed has direct drive it might be 15 gear and the last 3 are when the cops are chasing you VS specing the truck to run in 18 all the time and the rear end for say 75-85 MPH. You will end up with a truck that spec wrong.

I would find out the direct gear for the 18 speed and see how fast the truck will be going with the 3.21s

 

In a double overdrive transmission like most of the 13 and 18 speeds of today are, it is gear 16 (or 11).
@Cowpie1 has a setup like this. 65 mph in 16th, only uses 17 and 18 when he really wants to move, with IIRC a 2.64 rear end.

Some people will claim a lower rear axle ratio (faster truck) will fall short in the mountains. I haven't driven a very tall geared truck and the math doesn't make sense why it would "be slower" climbing mountains. More downshifting, yes. Slower road speed? I wouldn't think so. Difference is where the torque multiplication happens. Fast rear end, less torque multiplied there, use smaller gear in transmission, is where it happens, and the end result of torque applied to wheels would be the same. Same thought process for the low end, and same again if the truck is to move a very heavy load where it will need all the torque multiplication it can get to move a very heavy load from a start or up a mountain.

 

I'm a little late, but gross up here for a tridem is 102,400 lbs.

Hope I made your day giving you the extra room.

I'm picking up my new used ride later this week. ISX 565, 3.55 ratio, 18 speed. Yes it will do fine in the mountains and the flat.

I'm living proof that low ratio can work. My last truck had 700 hp, 2250 torque, 18 speed, 22.5 tires, and a 3.21 ratio. Could out pull a Cat, and fly on the flat. I speant most of my time running Vancouver to Alberta, so I was mainly mountains all day. I still averaged 6.3 mpg pulling maxed tridems.

 

I've drove a retired H.O. Wolding cascadia. DD13 engine, 10 sp direct, 2.36 axle ratio. Drove an pulled no problem, at 80k. Even out in the Appalachian areas.