Wheel & Tire Definitions

Aspect Ratio- This is based on the percentage the section height is of the section width. This figure is expressed in percent form in a tire size description, such as P245/45ZR17, where the “45” is the aspect ratio.
Backspacing- This is the dimension from the wheel mounting face to the rear lip of the rim. This measurement is important for inboard wheel clearance.
Bolt Circle- This is the distance between each lug, measured from the center of one wheel stud to another. You measure from the center of the stud or bolt hole diagonally across to another stud or bolt hole (for four lug); for five lug you measure from one stud or bolt hole to the far side stud or bolt hole. This is another easy way to quickly check to see if “open box” or used rims will work on your Mustang.
Offset- The offset of a rim is the distance between the true centerline of the wheel and the mounting surface. This dimension is critical to suspension handling. Positive offset moves the rim inboard, negative offset moves the rim outboard. Subtract half of the wheel’s width from your backspace measurement to determine the offset.
Overall Rim Diameter- Overall diameter of the rim face, from lip to lip, not the diameter used for tire sizing, which is wheel diameter.
Plus Size- To get better handling people often plus size their wheel-and-tire package. This refers to increasing the wheel diameter while keeping the overall tire diameter the same as stock, effectively shortening the tire sidewall. The shorter sidewall and slightly wider tread improve handling and maintaining speedometer calibration. Plus sizing is described in 1 inch increments; a Plus One fitment is a 1-inch-larger rim than stock, and so on.
Rim Width- This measurement is taken from the front bead seat to the rear bead seat of the rim. This measurement is used to determine tire-sizing capabilities of the rim.
 Section Height- Section height represents the dimension between the edge of the rim and the face of the tread, also known as the sidewall of the tire.
Section Width- The section width of a tire is measured from sidewall to sidewall across the tread of the tire and is measured in millimeters. The first number in a tire size is the section width. Using our example tire of a P235/45ZR17, the “235” is the section width measurement of 235 mm.
Wheel Diameter- The wheel diameter is measured from the bead seat. To measure true wheel diameter of a wheel with unknown measurements, the tire must be removed from the rim. The wheel diameter is found by measuring the overall wheel diameter, then measuring the rim drop or depth of the bead seat, multiplying the bead seat measurement times two, and then subtracting from the overall wheel diameter.
When it comes to fitting larger wheels and tires, there’s a bit of math involved. For those numerically challenged, here’s some calculations that might help you fit those sweet wheels you’ve been dreaming of.
—To Calculate Tire Diameter:
2 x (section width x aspect ratio / 2540) + rim diameter= tire diameter
Example: P245/45R17 2 x (245 x 45 / 2540) + 17= 25.68-inch diameter
—To Calculate Effective Drive Ratio: Old tire diameter/new tire diameter x original ratio= effective ratio