Poor fuel mileage

Change filters, start getting fuel elsewhere, might have picked up algae

 

Kind of a loaded question!
How long have you had truck to map mileage? Did you change your driving habits, ie terrain, location, winter, wind... weight,,, running 85,500 now with a spread axle hopper running hills with 30 boost on turbo! Get my drift here. If I throw on the milk wagon vs hopper I get 2 more miles per gal. Sometime less on Friday evening if i'm running behind to get back home, tend to lay on the fuel pulling hills.
Did you change to a different wagon? "Tanker to something square"!
Tire size the same?

Ok answer these then people will give some insight on what may be an issue with the engine.

 

So 5.2 mpg vs your previous 7.5 mpg. Cold weather set in idling more?Still on the same route back and forth, give or take on your traveled path?
What's your gauges telling you on engine, running more boost then normal (or less if you have a leak in CAC), temps running higher, 'did you take your cylinder temps again with your temp gun to see if they have changed'?
Time now for the truck Detroit guys to get on with there knowledge.

I'll just stick to my knowledge of the 8v92 with a 2-1/2 to 1 twin disc clutch turning up about 2100 naturally aspirated. Dang find boat motor, leaked oil everywhere, but I digress; and had a flash back of my youth.

 

Air filter.

And are you in extreme wind?

That is definitely a dramatic drop off.

Sometimes a sensor in the aftertreatment goes out of whack that will cause problems like this as well.

Wind will absolutely knock the snot out of your mpg though.

 

Winter fuel