Help with Rims/Tires numbers?
 

I have a basic understanding of tires and the numerical designations assigned to them. (i.e. 255/40 17 -- 255mm width of tire -- 40 is the aspect ratio compared to the width of 255mm -- and 17 is the diameter of tire)

The Rims and the offset numbers are where it gets foggy for me.

My Turbo Twist wheels have the following numbers on them FRONT 7 and 55
REAR 8.5 and 50. What do these numbers mean?

And the OFFSET is a complete mystery to me. How do I calculate the offset of a tire/rim???

Thanks in advance for any education here. AND please dont tell me to "do a search" or some other smart answer. I did a search and the info I found was limited and confusing.

7 and 55 represents 7" wide wheel with 55mm offset.

Tirerack.com Tech Center:
TireRack.com - About - Tech Center

Tire/wheel Diagram:
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=95

Wheel offset:
Wheel Tech - Offset

Offset is basically the distance from the wheel's mounting surface (often referred to as the back pad) to the centerline of the wheel. As far as Porsche fitment is concerned, only wheels with positive offset are utilized . In the case of your wheels, the numbers 55 and 50 refer to the offset of the wheel. In simple terms, when calculating fitment, the higher the offset is the more inwardly the wheel will be positioned toward the strut. Conversely, the lower the offset is, the more the wheel will be "pushed" out toward the fender lip. It's important, however, when changing your set-up to carefully factor in wheel and tire specifications in order to determine the correct fitment (i.e. clearance).

Thanks Johnny and Ekam...makes sense now