A) Centerline - Middle of the rim.
B) Diameter - Diameter of the rim. Unit: Inches
C) Overall Diameter (O.D.) - Diameter of the wheel (rim and tire). Unit: Inches
D) Rim Width - Width of the rim. Unit: Inches
E) Offset - Distance of the mounting hub to the wheel's centerline Unit: Millimeters
F) Tire Width - Width of the tire tread. Unit: Millimeters
G) Tire Profile - Height of the tire sidewall. Unit: Percentage of the Section Width
All the math you will ever need:
1" = 25.4mm
Diameter:
Diameter will not play a significant roll in fitment as long as you stay within an equal (or close too) O.D.
Applying Width:
Convert the difference in width of your current wheel to the new wheel from inches to mm's. Because width is distributed to both sides of the centerline you will then divide that number in half. You now have the amount wider/narrower, in mm, that your new set up will be to each side of the centerline.
Cheat sheet (+/- 1mm):
1/2" = 6mm/Side
1" = 12mm/Side
Applying Offset:
Offset is calculated directly. (+) Offset moves the wheel towards the center of the car, (-) away from the center of the car.
Spacers act as (-) offset.
Choosing Tire Width:
Convert the tire width from millimeters to inches. Compare that number to the rim width. If you need a stretch for clearance run narrower. Want more meat? Run wider.
Choosing Profile:
O.D. is fairly available information and the goal is generally to match that number as close as possible using the desired diameter and section width. A good rule of thumb is for every (+/-) inch you will need a (+/-) 5 series profile tire.
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Armed with this information and a tape measure you can figure out any and all fitment questions.