H E L P >>99 4.6 bad gas mileage

[pancake]

I HAVE A 99 SUPERCAB F150 4x4 auto with a 4.6l the truck has 115k i bought it in early 2008 with 75k ...it has been a great truck..i am a die hard ford guy ..and i didnt buy it to get good gas mileage...but i just recently took a few 100-200 mile trips from home out on I-90 (65>70 MPH)and i decided to run the numbers to see what i am getting for highway mileage... i just about threw my calculator across the room when i came out with 11 miles to the gallon on the highway ..and 9mpg in the city now according to fueleconomy.gov my truck when it was new was rated to get 16 on the highway and 12 in the city and yes i understand those numbers are obtained under the best of conditions..no wind,no load,smallest tires possible but i was hoping to be a little closer than what i am the only mods this truck has is a K&N drop in cone filter in stock air box,265/75/16 tires(10 ply heavy truck tires...whick is the only load range that i could get the general grabber at2 in for the 265/16 size) rather than the stock 255/16s,and a new flowmaster 40 dual outlet muffler with dual 2.5 tailpipes that dump behind the rear wheels should i be happy with what i am getting and shut up or is there anything that i can do to get a few more MPGs

[jimandmandy]

Get rid of the K&N and carefully clean the heated platinum wire inside the Mass Airflow Sensor. Im surprised it hasnt thrown a Check Engine Light. What happens is that vapors from oil used on the filter combined with very fine dust that the filter doesnt stop eventually coats and insulates the wire, throwing the mixture off. A lazy O2 sensor may not be able to fully correct it. The other possibility is that the engine has been "chipped" by a previous owner.

LT all-terrain tires are going to cost you some mileage, but not that much.

[21411]

With what youve got im not supprised sounds like my old truck. After the tires and exhaust i lost my giddy up and never bothered to check milage. Best that truck saw was 22 mpg going 100 mph from joliet to st louis. Before tires it was low teens.

Did you gear the axles for the tires? Ive always regretted not doing that before trading it.

[pancake]

i did not regear the axles...i am not that mechanically inclined and didnt think going one size up on the tires would have such a big effect... on that note i posted this on another forum and someone said that i should have recalibrated the speedometer when i did the tires which i did not and that person said that because i did not recalibrate the speedo the mileage traveled may not be totally accurate which would make my MPG figures wrong

another person posted that i may have too cold of a thermosta in it which will affect the temp sensor and fuel mix

and as far as cleaning the M.A.F. i will try and get that done the next warm day that we have here in NY since i do not have a garage...but my question is what should i clean it with...will carb cleaner and a clean rag do or ..is there something else that would be better

[jimandmandy]

The MAF sensor wire is very delicate. Dont attempt unless you are comfortable with doing it. A q-tip and some alcohol is what can used. Normally, it would throw a Check Engine Light if it is real dirty.

As was stated by others, speedometer error may be what is really happening. You may be getting better mileage than you think. Worry about that first.