If you draw a circle through the center of the studs, the diameter of the circle will be 4 1/2 inches.
Here is Tim measuring bolt spacing: http://pics.bothner.com/2005/NCDec/NCDec083.html Or you could do it this way which is the preferred method. http://www.rsracing.com/tech-wheel.html Notice that on the 5 lug wheel, that you measure from the CENTER of one bolt hole across to the OUTER EDGE of the other bolt (or hole) as necessary. Look at the pics real close and you will see what I am talking about. This link will also tell you everything you need to know about critical rim measurements.