Ive got an 06' 4wd V10 It has a leveling kit and some BFG AT's 315/70-17's When I first got it with stock tires it seemed to be getting 12~14 around town. After the leveling kit and tires it got 10~11 It seems like the mileage has progressively got worse. I see 8~9 around town now. I pulled a 7k box trailer over the weekend and got 5mph mostly flat ground with some small hills.

The truck has 52k miles on it; is there something that would cause the mileage to go that far down?

 

How is the weather? I loose mileage big time in the winter cold. Yours is doing the same as mine. 8-9 empty and babying it. Once spring comes it will get better.

 

Maybe it is time for a tune up

 

I just noticed my mileage dropped to 7.4 per gallon. I checked my air pressure and they where all off, probally due to the cold. refilled to 40 psi and mileage jumped back to 10.9 per gallon. I was shocked. never realized how much air pressure could affect mileage.

 

I had the same problem, once winter hit my mpg tanked. Hoping it gets better when spring rolls around.

 

same here winter kills these motors. i cant wait to drop a cummins in my 02

 

40 psi seems pretty low for a 10ply tire and everyday driving. Are you using "D" tires?

 

Colder temps means denser air, which means more gas in each intake stroke.

But do go over the usual things, fuel filter, MAF cleaning, tire pressure, brakes dragging, that sort of thing.

 

You put tires that are over 3" taller than stock on the truck.
You didn't inform the PCM as to this change in tire diameter.
You odometer is off.
You have more than 52K miles on this truck because of this.
Your speedo is off.
Fuel mileage is not as bad as it appears.
JL

 

I'm running dick cepek fun country II's they only have a 50lb psi max.

 

What gears to you have? If you have 3.73s you might benefit by re-gearing to 4.30 or 4.55's for around town driving...

 

Its off a little, but not that far off; GPS vs the speedometer is about 4mph @70

My truck has 4.10's

Could it be due for spark plugs already?

 

Casey, some ideas here: https://www.ford-trucks.com/forums/1...-way-rich.html

 

I'd change them.

 

X2!!!

I have 285's on my truck and my speedo is only 2.4mph off vs GPS and that means about 1mile off for every 20 driven. Take a tank of 250mi per the odometer that means an additional 12.5 mi. Doesn't sound like a lot but if you filled up with 25gal thats 10mpg per the odometer and actual mileage is 10.5. Now thats figured out with 285/70 17 tires and you have the 315's which you report make your speedo 4mph off vs my 2.4 off so

I'd say just by estimation your at least 1mpg better than what you're calculating by what your odometer reports.

I have a DiabloSport tuner for my truck and it corrected the speedo by inputing the tire size but the odometer still records off. Now that I know this, no big deal but I did go through a time there for awhile where I couldn't figure out the mileage drop.

I now use an app on my android phone called "torque" Torque — OBD2 Performance and Diagnostics for your Vehicle that allows me to access pids, clear/check any OBDII DTC's, GPS speed, GPS trip, datalogger, and monitor instant/average fuel economy. Uses a bluetooth DLC adapter to communicate and is a pretty useful tool. DLC adapter was about $50 on ebay and the app was like $6 in the android market.

Oh and unless it's misfiring, I don't think a spark plug change is gonna help you.