Reading Wheel Sizes: per discountTiredirect.com
Example Size: 15x7 4-100 40S
15 - Rim diameter in inches.
7 - Width of rim in inches.
4-100 - Bolt Pattern.
4 - Number of lug holes in wheel.
100 - Distance between lug holes (in millimeters).

Some wheels have multiple bolt patterns.

15x7 4-100/108

This wheel fits cars with both 4-100 bolt patterns and 4-108 bolt patterns.

40S - Offset and finish

40 - Measures in millimeters how far back or forward the mounting surface (face) is from the center of the wheel.

S - Silver Finish.