HELP any help or info for poor fuel mileage fixing!!!!!

[bravenight]

Hey Guys gals, I have a question, or problem ...

My truck here has become a pig on fuel, It is consuming way way to much fuel, compared to what it used to consume years ago

I have a 1996 power stroke I believe it is 1st gen 7.3
It has only 1 mod done and that is the 10 k resistor , but i have it on a toggle switch and i only turn it on when i have a heavy load on.
The turbo is fairly new on it, not that many miles on it when i replaced it last.It has 410 gears front and rear . but the mileage I used to get when i would head down to the coast has changed drastically. when we were forced to run ULS diesel it dropped big time compared to LS fuel.
but comparing to the fuel mileage from when it was first introduced.
so here is what i am now consuming.


OK I this past Friday did a road trip. I put on 450 miles or 750 km
I used 200Litters of diesel

is it possible that my fuel pump the one near the fuel filter i am assuming is the high pressure one. is it possible that if it looses its capability to produce stock or more pressure would it decrease the fuel millage?
what are some other things i should look for that would be a culprit of producing poor mileage like what i have now?

the air filter is new, fuel filter is new, i have no muffler straight pipe, have the down pipe mod,tire pressure is all up and im running nitrogen in the tires.
I also have 35 inch tires on the truck when i first put them on , my fuel millage increased ,and i put them on due to the 410 gear's.

other then that i am stumped on what to look for or what to test for or how to test the fuel pump or what.
when i first start the truck up im told its the dual mass clutch. but the truck shudders or rattles away, or I dunno how to explain it, but i was just told it was the dual mass..
any hows any and ALL help would be greatly appreciated.
thanks again guys