9-10 on the highway is pretty bad - My '01 in mixed-driving gets 10.6 regularly back and forth to work, 22 miles each way. That's NOT all highway - that's mixed - about 3/4 of it is pretty much light-to-light.

BUT - you have a dually - those fenders do stick out pretty far and do put up some wind resistance., and the extra tires do increase the rolling resistance.

Like others have said, the tire size is different, so you are actually going further than you think. If you clock 20 miles, and the original tires were 215/85R16, with 235's, that works out to 20.85 miles actual. Not a big difference. At 10MPGs, that works out to 10.43 actual.

I'd clean the MAF, change the fuel filter, check the fuel pressure (28-30 idling, 38-40 under heavy load or with the engine off/key on). If the air filter hasn't been changed recently, do that too. Even when they look clean, they aren't necessarily.

 

My 08 4x4 x cab 250 was getting 14.5 when I got it and now it consistently gets around 10.3. That's me cruising at 60 and taking off like an old person. Something has to be wrong with mine cause I haven't changed anything and I want 14+ back again.

 

I hauled our 9000 lb travel trailer up to the campground yesterday afternoon (we have an '06 CC). It's about a one hour trip and mostly highway. According to my Scan Gauge ave mpg (I'm not sure how accurate it is), we got 7.5 mpg hauling the TT and 13.6 mpg on the way back without the trailer.

 

It probably needs some new Motorcraft plugs if you haven't already done them.

 

Ok should i take it back to the ford dealer or should i attempt it myself? I can change spark plugs done it a bunch of times, but im leary of these aluminum heads, if i screw up its on me, on the other hand if i take it back to the dealer its on them. Or have all these siezing issues been resolved by now?

 

Ford dealer would cost you a pretty penny, especially since they take the cost of removing a plug(s) into consideration up front.

I've seen Srmastertech do this on an '04 F150 and here's how he did it:



The theory behind it is that the impact wrench seems to gently tap it loose instead of the instant breaking force of a socket wrench that breaks them off in the head.