2.64 with 13 speed double-over & 22.5 LP

bit of low bed here and there but mostly a fridge, didn’t really look that close at your Av to notice the rack honestly

 

That’s a huge change in my opinion going from 3:58 to 2:62 personally I think to big of a change

 

Why did you swap it out?

 

It makes a huge difference especially when it’s windy or cold.

 

Make sure you have the speedometer corrected using Cummins Insite and run that truck in 16th. The purpose of the direct drive 10 speed and 2.64 gears is to make the best use of the 1:1 gear ratio in 10th gear because that is the most efficient ratio. To utilize direct 1:1 in your transmission means you need to run in 16th which is the direct drive 1:1 ratio.

Only use 17th when empty and you are really in the mood to make up some ground. Your ISX likes to work and it is not a "downsped" engine. When pulling hard keep the RPM around 1600-1750. When cruising under light throttle load 1450-1550 will be fine for the engine.

Get yourself a boost gauge so you can see what is going on. More boost, more fuel. It will take some time for you to learn what this engine and driveline combination like for the best fuel economy.