Wheel Size
 

This may be basic (but I've never read my wheels before). On my front OEM wheels, on either side of the valve stem, the front say 7 55 and the rear say 8.5 50. I'm sure the 7 and 8.5 are the wheel width but what do the 50 & 55 indicate?

Rim Offset. Here is an excellent link: http://www.1010tires.com/WheelOffsetCalculator.asp

Thanks. I never read my Cayenne wheels. It's obviously different when the front and rear wheels are different.

You'll commonly see the German abbreviation for offset, ET or et. The diameter and width of a wheel are in inches while the offset is in millimeters. For example, 18x8.5 et 45 is a 18" diameter, 8.5" wide wheel with 45mm offset.