Needing help from 5.6mpg to 6mpg+++

Sounds like you have very low rear ends like around 390s or 370s. The only way you're going to get fuel mileage is slow down to about 65 around 1300 rpm. Only other option is change out the rear ends but that is expensive and may only get you a few 10ths.

 

Try using a synthetic oil,I switched to Amsoil and the truck runs better, with a flatbed the wind is a big issue, get a scanner that plugs into your ecm and keep an eye on it as your driving and you will see what your best RPM is

 

Return on investment there is so long its really not even worth it unless its time to rebuild the rears anyways.

 

Freightliner? Isnt there a red and green mark on the rpm gauge? Forget about speed keep the needle in the middle of the green. Use cruise when ever practical and SAFE to do so.

When I said forget about speed, I meant driving by rpm should keep your speed legal or close enough. Sounds like your best place is 9th at 13rpm and 10th at like 9-11.
Bear in mind I have been driving 13's for the past 15 years or so. So I might be off cause you have a 10

 

You want to drive in the highest gear while on the freeway. Don’t downshift a gear to drive slower. The higher the gear, the more fuel efficient your truck will be. I would recommend driving at 1300 on the highest gear as long as you can still deliver your loads for the week. By the way, everyone’s mpg lowers during winter.

 

Is your turbo boost on point ?