I have an '05 5.4L XLT SC. It only has 1600 miles on it. It's stock, outside of the 33's. Right now I'm getting anywhere between 10.5mpg and 12mpg in town (not much hwy...no figures). I realize that I won't see a huge jump once she breaks in but, when should I expect a change? I drive reasonably and only occasionally stomp on it. Also, what are you guys seeing for in-town mileage?

Hoping to see that "14 mpg city" seen on the sticker,
Dan

 

Have you adjusted your speedo for the 33's? Or are you taking thier larder diameter into account? If not then their is about a 1-2 mpg difference from stock size, snd that would put you into the mpg range that about all of us see with the 5.4's.

 

what gears do you have and what tire came on it stock?

my '03 came with LT245/75R15 and I jumped up to a LT255/85R16 making the speedo, and odometer off 10%, meaning I'm getting 10% better than it shows doing the math at the pump.

Now if this is the same for you, you would be actually getting 12-13mpg. Now if you went wider on the tires that could cause worse gas milage too, but your only talking the loss of 1 mpg.....minor.

Big tires look cool, but they kill gas milage.

Does this make sense?

 

I never really thought of the tires (wow, and I'm a tire guy! ). So, if my speedo is off 1-2.5 MPH and I get 300 miles to a tank....How many "extra" miles am I getting? Can't be that much. Like 3% or something. I'm lousy with math. Anyway, pretend I have my OE tires on....I still got in the 11mpg ballpark. A buddy of mine said his mpg improved at 10K and he's stock. Thanks for the feedback...look forward to more.

Dan

 

Without the dealer recalibrating the speedo you have to figure it by finding out the previous tires revolutions per mile and the divide that by the new tires RPM.

Example:

original tires on my truck are BFG Radial Long Trail 265 60 18's...685 RPM according to BFG.

New BFG All Terrains at 285 65 18's turn at 634 RPM

So 685/635=X sorry don't have a calculator handy

X times odometer reading/miles traveled=true distance traveled, then you divide by gallons used.

 

ive got 33's on my fx4- got re-programmed by the dealer for $30- i can tell you that my mileage isnt getting better yet...highway im getting about 15 ish...in the city/town, im only getting 9-11 depending on my methods... fyi, the new 2005 consumer reports issue rates the f150 5.4 as only getting 9mpg average on in town driving, and 16 as a high on the highway.. (compared to the Ford/ epa estimates of 14/17) i've given up on getting great mileage with my toyos and 3.73 LS diffs. hope that helps a little for ya,
Justin

 

FWIW, my '99 V6 really improved the mileage and performance about 11,000 miles. I know they manufacture the engines with tight tolerances, but I really did not think it would take so long to 'loosen up'.

Also, not only will the taller tires screw up your speedo, but they probably have a lot more drag due to the increased width and sticker rubber. Even though you are turning lower rpm, your actual mileage will usually suffer with wider, stickier tires.

Finally, there is a wide range in fuel quality (energy content) around the country. Ford trucks really seem to get lousy mileage on lousy fuel (like in California). You did not say where you live, but if you get 'clean air' fuel you will probably get worse mileage than somebody who lives in an area with good fuel.

 

The "other thing" you should have noticed is a corresponding decrease in rpm per increase in tire size without changing gears. I.E. if you were going down the highway @ 70 mph on stock (short) tires you were probably in the neighborhood of 2k rpms on the tach. If you increase the tire size @ 10% then you would decrease rpm respectively. Now, is this a big deal? Yes and no. Dropping 200 rpm is not significant by itself, but you are shifting to the left on the torque curve which means you would need to give it more gas to accelerate; affecting milage, and when you reach the desired 70 mph at highway speed you are also makin less HP. Lets say it takes 70 HP to cruise at 70 in stock trim because of drag (wind resistance). Further; if the truck is now only making 65 HP at that lower RPM at highway speed then you will need to add more power (fuel) to achieve the same speed. And, a taller vehicle creates more drag and the larger tire may have more rolling resistance at the air pressure you are running.

 

Chassismaster said it very well.
The 'i didn't think' should never be done on anything of this cost and magnitude; it cost to much to find out the hard way.
If fuel milage is a "great concern", then you can't afford to own a truck of this size.

 

I'm not really sweating fuel economy. More of a curiosity. Just to see if a broken-in engine will make a significant increase in mpg. Naturally, I'll be excited when/if my MPG goes from 11 to 12 but, it's not life or death. If I had a goal for my F150, it'd be 350 miles on a tank in town!

Thanks for the info guys.

P.S. I'm from central Oregon, where we have yet to care about air pollution. I do my best to stay away from Arco (Aquagas).