I'm getting some conflicting information about this from the people in my usual gaming group and I want to get some solid confirmation on this so there's no confusion.
When wearing a suit of metal armor made of metal, does it effect your movement speed in any way?
Would a character with a base movement speed of 30ft. still be able to move at that 30ft. while wearing a Mithril Breastplate or would he still be reduced to the 20ft. movement speed as noted on the armor table?
What would be the movement speed of a character in Mithril Full-Plate armor if said character's race has a base movement speed of 30ft.?
Mithral armor affects the wearer as armor one category lighter for purposes of movement. So mithral breastplate affects them as light armor (so not at all, for movement speed), but mithral fullplate would function as medium (20 ft. speed for a 30 ft. base character, but able to run at 4x still).
If you don't have the armor's actual proficiency, however, you take the (modified) armor check penalty on attacks and whatnot, even though it counts as a category lighter for movement and other restrictions.
Core Rulebook pages154-155 have the stats on mithral.
Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.
I have bolded the relevant two portions.
The first bold indicates that for the purposes of movement mithral armor is 1 category lighter. IE: Medium moves at the speed of light armor and heavy moves at the run speed of medium armor.The second bold indicates that you still require normal proficiencies for this. IE: if you are wearing mithral heavy armor you must be proficient in heavy armor or else will take penalties.
Exception to this: Elven Chainmail counts as light armor for the purpose of proficiencies but costs an extra 1000gp over mithral chainmail.
- Gauss