Truck: 2004 F150 super crew FX4.
78k miles, stock sized Michelin LTX on factory 17" wheels.
No mods, no level, nothing.

I obviously did not buy this truck for fuel economy concerns. However, I'm averaging 10-11 mpg no matter what really combined city/highway. The last long highway trip I had it averaged 12 mpg running 75 mph. That tank was completely highway.

I don't try to hot rod around, no point really.

What should I check out or even replace first?

-o2 sensors
-clean maf
-change fuel filter

Thanks for all insight.

Lorne

 

I went from 230/70/17 ( stock 2x4 tires ) to 245/70/17 and I also went from 17 MPG to 13 MPG, AFTER I did a little smoke show.
Theory,

The LSD is locked in, and is the cuplrit, going to flush and refill mine, if both wheels are locked in all the time ( I think they are ) that might explain for the loss.

My dad who is ASE cert, says that might be the cause of my MPG drop.

 

All three for starters. At your mileage you have some maintenance to do. Lazy O2's will play havoc witheconomy all on their own.Ensure yer parking brake is not binding.

Within the last few months there have been a ton of "low-economy" and "maintenance" type threads. Familiarize yerself with Advanced Search and have a look at those. There is a LOT; You'll need a few of these ==>

BTW - 75mph is not an economical speed in these pigs

 

I'd also change the spark plugs, but be aware of the sticking and breakage issues.

 

Agreed about the speed. The faster you go, the harder nature is trying to slow you down. There is a significant difference between 65 and 75, and an even bigger different between 60 and 75mph. When I used to commute on the interstate to college, I would drive 70mph but I noticed I gained at least 1-2mpg just by dropping it to 60mph. The difference in time arrived at your location depending on speed difference is a moot point.

Check the plugs, air filter, fuel filter, O2 sensors, I know i've had some go out without so much as a warning, do the routine maintenance to see if it changes anything. Also, do you have the 4.6 or 5.4L?

 

He has the 5.4L. Can't get 4.6L in an FX4.


OP:

You are driving a 6,000lb vehicle with approximately the aerodynamics of a brick wall with a big V8 and trying to maintain high speeds.

 

Thanks, didn't know the 4.6 wasn't an option.

 

your right foot has more control over mileage than anything else. a tune up certainly wouldnt hurt if your truck is due for one but i would also look into your driving habits as well. as stated earlier, 75 is not good for mileage, drop it back 5 or 10 mph, coast as much as possible and dont stomp on it from a stop. also the weather can have a big effect on mileage too. is it cold where you live? do the gas stations use a winter blend fuel? both will have a negative effect on mileage. its cold here and i have seen my mileage anywhere from 10 MPG's to 14 MPG's depending on my right foot.

 

buy a smart car

 

No way - a VW TDI.