Progressively Low fuel mileage

[Butch59]

Hey everyone.

We have a 2017 6.6 diesel duelly with 74k miles. At 73k we had 8 new injectors put in, oil change, new fuel and air filters, values cleaned. 6 new tires properly pressured. We are pulling a 37ft 5th wheel, also with new tires properly inflated, no water on board. We drove 9k miles last year out west and got on average 11mpg with the 5th wheel heavier than this year. We have gone 1k this far on flat roads, averaging 60mph and we are steadily falling in mpg. We started at 11.2 and now at 10.3. what should I be looking at? We are heading west again for another 9k trip, we are only 1k into it. Do injectors need break in time? Should I have the oxygen sensors looked at? 

Appreciate any input!!

[richard wysong]

it could be just different fuel, I notice on my annual road trip to sunny Fla that the mileage is sometimes less or more after refueling. I would check to see if you have a brake dragging as well before throwing parts at the engine

[Grumpy Bear]

There are things that are not even truck related that can account for this. Differences in traffic flow, wind direction and strength. Air temperature. Number of construction zones along the route. Change of average road speed. Even small. It isn't always a tune thing. Can also be non drive train related. Change of alignment is a big one. New tires? Two data points is a really small data set to base a conclusion on don't you think?