All wheels are marked with numbers that signify many different things about the size and type of wheel.
For example, a wheel may read: 20x10J ET35. The 20 refers to the diameter of the wheel being 20 inches wide and 10 means that the wheel is 10 inches wide the other direction. The J refers the shape of the specific flange for that wheel design and the ET stands for einpress tief, which is German for offset, which on this specific example is 35.
Another confusing number to understand is the P.C.D., or pitch circle diameter. Measured in millimeters, the P.C.D. is the diameter of the ?circle? that the bolt holes for a wheel create. These bolt patterns can be found in four, five, six, or eight and vary from wheel to wheel and car to car.