Cat 3406E Question! MPG Poor at 3.5-4 mpg.

I have a Cat 3406E. Year 1994 . The motor is sitting in a 379 Pete. Transmission is a 15 Speed with 3.73 rears. Tires are 24.5 tall . The motor has a 550HP and 1850 Torque tune, new fuel pump , filters air and fuel . We are getting 3.5-4.0 MPG. The turbo side of things as follows , we tried running with the waste gate and without same results . Boost with the waste gate is around 30 without waste gate is around 40-42. Also truck hauls around 80-88k per load .
Question is : We heard someone say adjust the waste gate for lower boost to get better MPG . Is this true ? What can we do to get any better numbers .more boost less boost? Thanks guys

 

Speedlimit

 

So could adjust waste gate for lower boost pressure?

 

So I’m local , I run from one mill back to reload 75 miles apart . Back and forth , some hills yes but mostly 4 lane hwy, 65 limit 90% of the 75 miles . So with that Cat motor keep it in high rpm’s? Don’t let it lug at all? Because I’ve tried that before and no difference, but I’ll try again

 

But I can run high rpms and boost still at 30 that 15 speed is spread out