To better uderstand the vertex behaviour try this little experiment: scale down the default cube to something like 0.1 BU, then create an armature with two bones, one on the left (X = -1) and one on the right (X = 1) of the cube.
Assign a weight of 1 to all the vertices relatively to both L and R bones.
Then in pose mode move the R bone by 1 on the X axis: the cube will move on the right by 0.5 BU: this happens because all vertices are influenced by all two bones: one is telling them to move 1 on the right, one is telling them to stay still; the average of the two instances will result in half the movement (1 + 0)/2 = 0.5.
Your wheel has the same problem, because is weighted to 6 different bones: the correct approach is to have it weighted only to a single bone that will have to perform all the movements (i.e. its Y rotation will be the rolling, its z rotation will be the direction, its location will be influenced by the complex mechanism of the shock absorber, etc ....)