i bought a 1 owner 1997 ford f150 4x4, it has the 4.2L 5 speed v6 in it..i must say it really surprised me with the tq and power , the epa rating is 20 mph, im getting 12 mpg...i noticed that the other owner had installed a air hog air filter..i plan on replacing that with a stock one..cause on a fuel injected motor ..when you add more air..the computer adds more fuel..and my biggest question is just how big effect does the tire size have on gas mileage..i ask cause the sticker inside the door says the truck should have p235-75-r16 tires and it has 265 tires on it, i found a tire calculator on line and it says that there is 2 inch difference in the 2 tires.., could this be as easy as the tires ?

 

Recently motortrend april 2007 published an article about the epa revamping the way it determines fuel economy in vehcles. The epa is still using 1970's standards when measuring fuel economy. Few drivers cruise at 55 anymore and city drinving has increased substantially. Expect to see a 20% reduction in epa fuel estimates across the boards in 2008.
Your 20mpg 4.2L will realitstically make 16mpg average mpg.
As for tires, maybe.....
My 2000 4.6L 4spd auto averages 16mpg. On the highway the highest I have achieved is 20 (19.7)mpg and that was this last november on a road trip to virginia with my wife son and luggage in the vehicle averaging 70 with no ac for the most part. I run 265/75's r-16 as well. Yokahama Geolanders. The best tire I have EVER used. Quite, superb handeling and amazing tread life! 235/75 is the standard for 1/2ton f-150.
I would expect you to get a bit better gas mileage if you are keeping it under 70 and not over revving when accelerating. Its a truck not a race car.
how many miles do you have? DO you still have original plugs and wires? Factory air filter?
NOTE: on the fule air metering. The acceptable tolerance for fuel/air is +/- 3%. If you get outside this range, the computer throws a code and expect to see your check engine light come on. This may happen with the k&n or air hog. I dont know it the air hog is oiled. The oil can coat the MAF sensor throwing the dreaded P0401 code.
Other things to cause the stupid code:
MAF bad
O2 sensor out (dowstream or upstream, cant remember)
DPFE sensor
EGR valve or regulator
ELBOW valve located on the back of the intake manifold behind the throttle body, will crack or melt
Intake ports below throttle body ports clogged with carbon, requires removal of throttle body and extensive cleaning
Possible intake manifold leak

I have fixed every damn one of these (except intake manifold leak cause I cant find one) and I still have the stupid light on. It has cost me over a thousand dollars. I'm a little pissed at ford on this one. Other wise I cant complain.

2000 F-150 4.6L Windsor
Hypertech 93 octane power tuning
k&n filter
151k

 

i checked my speed o ..it is a full 10 mph off, the truck has 150,000 miles on it..im not sure on the plug and wires ..i have only had it a few weeks ..guess i can just change the plugs and wires and see what happens