Proper gearing

Changing to a 13 speed is not gonna change your final ratio. The 8LL is .73 in high gear. The 13 is .73 in high gear as well. They do make a direct 8LL model but with 3.90's and 11R 22.5's you would be tached out at 60mph.

Really changing the rears is your only option

370's will drop you about 150 rpm give or take and 3.55's will give you another 150 or so. But with 3.55S your gonna start to notice a loss of that race car feel

 

Swap the gear boxes, say a 3:50's to say 3:70 range, or there about. Should get you that speed you desire and keep it in range. They have gearing calculators out there, might want to Google that. The issue is your rear ends, not the transmission.

 

A good ecm tune from a specialist, some free flowing Donaldson’s like the m1000463 and a power harness would probably get you a nice gain. There are some posts on here about tuning for that motor and who to use, those mufflers are cheap at the TruckPro $100ish, and you can make that harness yourself for a few cents if you can solder decent.

That muffler was worth a few tenths on its own on a 2007 ISX 475.

 

I’d probably just put some tall 24.5’s on the drives.

 

^^ x2

 

Nice numbers on that truck, BTW. The only thing that's eating you is the highway cruising, right? Well, except for the taller rubber, there's no easy fix. The best fix would be a 2 or 2 speed auxilliary on the back of your transmission, but that's $$$$$. It works like the transfer case on a 4x4 pickup.

 

Tall 24.5's will gain you 3 MPH at the same RPM.

Roadranger calculator: http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

 

3.90’s and tall 24.5’s.

 

I like the little display that says 7.0mpg.
I think that’s what the dude is looking for, 65mph with 1500 at the tach.