[thstone]

It is completely normal for a tire to rotate on the rim (to some degree). With that being said, 360 deg in 20 mins seems somewhat excessive.

And yes, this could easily cause an out of balance situation. The wheel and tire are balanced with the tire at a certain location. If the tire was to creep around the rim, then the weight might not be in the correct location in relation to the tire causing an out of balance condition.

A better way to check this is to mark the tire at the location of the valve stem and then check later to see if the mark on the tire has moved in relation to the valve stem.