14 Mack, 4.6 mpg? Can I get better mileage with it?

I recently picked up a 2014 Mack CXU613, 445 HP 12.8 liter MP8 with the MDrive auto shift. Bobtailing it gets 9-10 mpg as normal but as soon as I hook up a loaded trailer I'm only able to get 4.6-4.8. It doesn't really matter how heavy it is or if I slow down from 75 to 65, it consistently always gets 4.6. I did get all my filters and oil changed and it brought it up an extra .1 or .2 mpg, but still. When I'm running 70 mph it's at about 1530 rpm. It's a midroof and I'm pulling a reefer so I've been looking into an air dam but I'm not sure if that's going to help much. I don't know how to tell the gear ratio but I don't think this truck was set up for long haul in the first place, it also has a 2 line wet kit on it. Any help or advice would be greatly appreciated.

 

Another thing to consider also, the truck is physically deleted, it still throws a Def fault code when I turn it on but doesn't try to regen or derate.

 

I can only get decent mpg when I'm running about 50-55 mph, but I run tight schedules so that's not possible, I get 30 psi boost at the most, I was told that's average for this truck but I'm not sure about that.

 

Reefer

What kind of gear ratio should I be considering? I've tried to learn about them but it's a little confusing for me, I called mack and from the vin they said it was at least stock 3.2x but it's 600k miles so possibly was swapped. I'm running 22.5 brand new tires that came with it so I don't think I'd want to swap tires until they're used up yknow?

How would I check the current gears anyways?

This is all just very strange because from what I understand the mp8 is basically a d13 right? And I've driven old volvos with more miles and much worse condition and they still get 6-7 mpg.

 

What speed and rpm are you running?