i think it tries to find some specific bump on the mesh to determine the primary and secondary one for baking. When the model is perfectly symmetric and the low-res isnt a much more difference, it may pick them up in the wrong order and thus trying to bump it inside out (like he would take it more like a cave segment than a tire - doe i think there should be a way to choose witch is primal to solve this problem, i saw that on some blender vid. at least ) - Also, baking is more for artistic stuff - like if i would do some Maya paint-brush buildup to extreme levels and then decimate the model to 500-1000 verts, on witch the normals would be baken - adding texture afterwards accordingly. ... other way is to do an detailed well fitted texture and use pixel-area processing tools like CrazyBump.
Doe, you could try something else - make the tire for baking a little bigger than the prop for texturing - make all the fine deformations on it and after baking, it should be able to suppress the normal map intensity so that the size-difference made effect would be suppressed (maybe it wont be - not sure if the program checks surfaces on a greater scale and suppresses this already somehow - guess it is tool-dependent), while fine detail prevails. Doe, not sure how it will go, needs to be tested.
... yes, you can send me the file and i can look onto it, doe, splitting UVs will be little bad for all the other vehicle parts.
