kenworth t660 mpg problem

i got a 2008 kenworth t660 455 cummins isx 10 spd 3.36 ratio hauling reefer from cali to florida on i-10 weight 70k-75k speed 73-75 mpg 5 miles per gallon i know i could get better mpg by reducing my speed but is this normal if not how can i fix this. I've seen t660's do 6.5 miles a gallon easy

 

sounds speed related to me
62 mile per hour co truck
would probably be 7.5 mpg
on that run

 

http://www.thetruckersreport.com/tr...3-cummins-isx-cm871-technical-discussion.html


Have fun reading..... Then roll up your sleeves and clean the soot out of that motor. Rawze has vidios too.