I'm going to disagree, and say that 17x9 +52 will suck in 10mm more than a 17x9 +42. The inner lip will also sit 10mm closer to the strut housing.
Positive and negative offset are measured from the centerline of the wheel. So you're only looking at the numbers of 52 and 42. In this case, the lower number being "more poke".
(
http://www.tirerack.com/wheels/tech/...jsp?techid=101)
It only works in the above example because the width is the same. Change the width of the wheel, and the centerline changes. A 17x8 +30 is the same poke as a 17x9 +42, but the inner lip on the 17x9 sits 25mm closer to the strut housing.
Very simple concept, highly confusing in text
