Terrible Gas Mileage
 

I know not to expect great gas mileage out of my 2004 FX4 5.4 triton. I have larger tires and a lift kit but the past few weeks I have been getting 7.9 and 6.7 miles to the gallon. (measured by the odometer and amount of gas put in) Is there something wrong with my engine or could it be an odometer problem? Any info will help/

Any rattling from underneath? Have you noticed a decrease in power? Either upon take off or maybe while gassing it on the Highway?

no noise, everything seems to be normal plus i just changed spark plugs and COPs.

Winter blend gas will cause some decrease in mileage. What were you getting before? With the lift and bigger tires, that doesn't help with gas mileage either though. For what its worth, my trucks gas mileage has gone down in the last couple of months as well. I live in Georgia and could probably go all year without the winter blend but they roll that out sometimes around October anyway.

How large of a lift and large are the tires? Have you corrected your speedometer for the tire size?

I have 35-12.5-18 tires with a 6" lift. I'm happy summer fuel

With summer fuel getting 10 mpg. Winter fuel high 8's :(

Regear 4 56 to get some mpg back

Ya re-gear would be my suggestion as well and winter blend will kick you in the crotch every time! I tend to run 91 Octane in the winter and 89 in the summer to compensate for the "blend"

You don't say what your mileage was previously. I just had the same thing happen to me. I was averaging 14-16 mpg with my 5.4. Lately, I have been lucky to get 11-12 mpg. I checked the front hubs to see if they were disengaging when the truck was running and they were not :glare:. I pulled the IWE vacuum hose off one of the front hubs and found no vacuum when idling. I have good vacuum at the solenoid connection coming from the manifold but nothing beyond the solenoid. When I applied suction to the lines feeding the hubs, the IWE's disengaged. Turns out the solenoid was bad. I picked one up from Oreilly's for about $24.00 and now they work again. I haven't really run a tank through it yet to check my mileage but it seems like it is better than before. Your mileage will suffer if your front end shafts are spinning all the time.
It is really simple to diagnose if the IWE is disengaging properly. Jack up one front tire off the ground. With the engine off, spin the tire by hand - you should see the shaft spinning because the IWE is engaged as it should be. Next, start the truck and spin the tire again. The shaft should not turn - if it does, you have a vacuum problem with either a leak or a bad solenoid.
Good luck with it