Need help w 3406e mileage

I need help from you guys running cat motors in your kw's. There aren't a lot of internationals running cat motors out there anymore so I'm hoping some KW owners can help me.

4.7 MPG!!!!!!??????
The truck is a 97 INT 9300 set forward axle flat top with a Cat 3406e and 13 speed high torque eaton tranny. When I was working the truck in mud and short runs I never thought too much about its terrible fuel economy. However, for the last 2 months I have been running from Indianapolis to st. Louis and back every day. All interstate pulling a van with aero skirts. I'm not driving over 67 at any point of the trip but I am averaging between 4.7 and 5.1 mpg on every trip.... 30k one way and about 17k coming back.... this is terrible. Running 11r22.5 tires. Can't go to low pro as the tanks would likely drag over tracks. 67 mph running about 1600 rpm. Have thought about lowering the rear end ratio to like 3.35 or something close to get revs down. Not a real powerful tune on it. It basically runs out of pull at 1600rpm which seems odd as I hear of guys running up to 1800 with these motors. I would never do that though just simply this motor doesn't pull beyond 1600. It was overhauled 400k ago. Tranny rebuilt last year. Help me figure out a way to get this old girls mpg up so I can justify keeping it.

My thoughts have wondered into these areas so far.

Reduce gear ratio

Add aero wing to the flat top

Retune the 2 month old ecm

Add balance masters to wheels (reduce vibration and rolling resistance)

Maybe convert to 6x2 with locking rear?

What do you guys think would work??

 

I'm thinking it hasn't been done in about 200k

 

That is where my mind was leading me also. Tks do u think gears in the low 3's would be too low?

 

Bought a new ecm from CAT in Fort Wayne when a sent the old one to the scrap pile

 

Short***

 

I lied 1

I lied 100k