Tires: Smaller tires on 4wd truck, 4wd vehicles, rim size

Question
We have a 2003 F150 4x4 Supercrew with P255/70R16 tires that need to be replaced.  In order to save some money, we were going to use  P245/75R16 tires.  I have seen articles which state that you must use the same exact tire size on 4wd vehicles, others state handling, braking and speedometer readings will be affected and yet others say it's fine to use smaller tires as long as the rim size is the same.  
Can you clarify what happens, if anything, if we "downsize"?  Does 4WD matter in this case?  What sizes can be used? Any and all inputs is appreciated.

Answer
Steve,

For practical purposes a P255/70R16 and a P245/75R16 are interchangeable,  This does NOT constitute downsizing.  They are within 1 1/2% in diameter, so the speedometer and odometer are still accurate enough.

That is the limit to what options you have for tire size - except for going considerably wider and that is going to increase the cost!

4WD doesn't matter so long as all 4 tires are the same.