I am very pleased with my F-150 but have an intellectual couriosity about my gas mileage. I have documented every tank of gas in the last two years I have had it. I am considering a 2.5" leveling kit, 3" add a leafs, belly pans, tall narrow truck tires and re-gearing.

Specifications:
2005 F-150
5,400 lbs empty, 5,500 with my tools
two-wheel drive
Custom 3" single exhaust
Magnaflow 3" muffler
Ford 9.75" rear end 3.55:1 gears
275x65x17 tires
steel wheels
4R75E transmission

My fuel economy:
10.5 city overdrive off
13.8 higway (40-55 miles per hour) overdrive off
16. interstate (65-85 miles per hour) overdrive on and off

 

Greetings from North Mississippi
I was initially curious about my actual fuel economy when I bought my 07 XL.While it does have the 4.2 v-6 I was interested to see what the mileage difference between it and its predecessor..a 97 XL with the same powertrain.
You might want to look into the ScanguageII from Autozone.While I do NOT work for AZ,its one of the best purchases ive made for the Ford.
It plugs into the OBDII port,and when set up will give real time mileage,as well as average trip mileage.Also will double as a tach,and a whole range of engine parameters.Not to mention it will scan codes,and reset them as well.
I bought the optional rear view Mirror mount,and it looks factory.
You would be surprised at the real-time readout...I "adjusted"the foot,and can average around 20+ city just by watching the unit.
It will set you back around 200.00 with the mount,but youd be amazed at the info it gives you.
It has paid for itself many times over.

 

I'm curious as to why you seemingly run around with O/D off. Can you explain that? It's possible this could be leading to a lower MPG than you would hope.
A level and larger tires are certainly going to decrease your MPG.

 

I generally drive in the gear that requires the least amount of gas pedal to maintain speed. So around 65 miles per hour it takes slightly less gas pedal to go than in third gear.

 

that is exactly what overdrive does. it is a higher gear, and therefore lower rpm's. it would help your mpg's if you left the overdrive on always. i only turn mine off when loaded.

 

Indeed, leave the O/D on. You should be getting around 13mpg with or without the leveling kit in mixed driving. If you add larger tires the MPGs will decrease. Unless you regear...then it might not get much worse depending on the gears. Anywho, leave the O/D button alone unless towing.

 

Why should I turn off overdrive just for towing? I have been told that the overdrive mechanism is the weak link of the modern automatic transmission. I am very intrigued by the idea of lower gears improving fuel economy. How tall of a tire can I go before reducing gas mileage assuming I match the tire diameter to gear ratio? I plan on going with tall narrow truck tires.

 

I Just completed a tow of a car trailer with a Honda CR-V on I-55 from Louisianna to North Mississippi.I have the 4.2 V-6,and while it was(needless to say) an interesting experience,I left the O/D on on flat runs.When pulling an incline,I turned it off to keep RPM,s up.Average MPG was around 17.
O/D is basically a direct link from engine to rear wheels.In essence bypassing the basic fluid drive of the transmission.O/D locks the torque converter.
It is generally recommended to leave the O/D on unless pulling a load.

 

Quote:

Originally Posted by Huntersbo

Why should I turn off overdrive just for towing? I have been told that the overdrive mechanism is the weak link of the modern automatic transmission. I am very intrigued by the idea of lower gears improving fuel economy...

Not really sure about 4th gear being the weakest link in the tranny...but here's an excerpt from the service manual:

Overdrive
Overdrive is the normal position for most forward driving.
The OVERDRIVE position provides:

• automatic shifts.
• apply and release of the torque converter clutch.
• maximum fuel economy during normal operation.

Second Position
The 2nd position provides:

• second gear start and hold.
• torque converter clutch apply and release.
• improved traction and engine braking on slippery roads.
• engine braking for descending steep grades.

First Position
If this position is selected at normal road speeds, the transmission will shift into 2nd gear, then into 1st when the vehicle reaches a speed below approximately 45 km/h (28 mph) .

The 1st position provides:

• first gear operation only.
• engine braking for descending steep grades.

Wikipedia has a good write-up on OD here: http://en.wikipedia.org/wiki/Overdrive_(mechanics)

The 4th gear ratio is .70 to 1