I'm new to having an F150 so I don't really know a whole lot about them, I have a 2005 F150 and I have a question about the overdrive, what is the purpose of it? when is the right time to use it? how do I know if it's on or off? do I save gas better if it's off? someone please help.

 

OD is the last gear in your transmission and is the tallest gear in your transmission so when using it you get the best mileage possible but you lose torque so it's not desired for towing as the truck will hunt for gears, if you have an automatic, when pulling up hills. It is a gear to allow you better fuel economy in normal driving/commuting.

If you have the 4spd AOD Tranny your OD(Also 4th) gear is a 0.7:1 gear ratio where 3rd is 1:1

If you have the column shift you should be able to press a little button on the end of your shifter to turn off OD and a light will iluminate on your Instrument Panel that says "O/D OFF"

 

You're best served with the OD when you're on a highway trip at a constant speed on a fairly level road. It'll increase your fuel economy by decreasing RPM.
Now,if your highway driving is on a mountainous highway, you're probably better off turning the OD off.
Secondary roads (especially rural areas), it depends on how curvy the road is and what kind of speed you can maintain. I turn mine off if I can't maintain a steady 45 MPH or more.
In inclement weather, I turn if off as well. Direct drive (3rd gear) will give you better engine braking. If I need 4WD, I turn the OD off.
And, of course, if you're towing you're better off not using OD.

 

I have the M5R2 (M5OD-R2, Mazda 5 speed with overdrive, medium duty) and I rock 5th gear all of the time. Depending on the speed, I can even use it when towing light loads. I tend to run 4th (my direct drive) when I'm fighting winds. Get the same mileage doing that but I'm not constantly shifting to keep speed up as there isn't a bunch of torque to push the truck through the wind in 5th gear.

 

leave OD on all the time unless you are towing, then shut it off. as soon as you shut the truck off and then back on it defaults to OD again.

 

^this