Fuel consumption vnl d13

Hello to all.
I have 2014 volvo vnl 740
It has I shift 2,64 ratio 430hp. Xe
My fuel consumption right now is around 5.3 mpg
Before was around 6mpg
I change injectors 150 000 miles also was around 6mpg from first day.
What can be a problem

 

That's horrible!!

 

I am pulling flatbed. So it's ok to be around 6.5 mpg. But not under.

 

look did u put a bigger set of injectors in or the same. They are rubber orings on them they could be leaking

 

Unfortunately I didn't do that by myself because I didn't have tools to perform that.
Truck dont have power.
Using fuel more than any other truck what I have. I am desperate. Idk what to do. And I am scared to go in shop because they will start changing parts and that will take me nowhere

 

Fuel mileage goes down in the winter cold air is harder to drive thru ,winter blend fuels grease and oils are thickened idle time increases !

 

Now I have never heard or seen the Dealer offering bigger injectors for the D13 motor but would be interested in seeing if that was even an option.
Also only seen the D13 hopped up to 550hp and 1900tq but again maybe someone has done more

 

When you drive and is under 35 deg outside does your engine temperature go below 170?

 

Curious on where this would go. Can you explain?

 

I had a bad thermostat mine temperature was going down to 130. Engine is not running efficiently at this temperature and the ECU is shooting fuel to warm up the engine.