Wheel size is measured in inches. first number is diameter, second number is width.

Offset is measured in mm. It determines where the wheel mounts to the studs compared to the center of the rim (as viewed from top). 0 offset mounts in the middle. Positive offset (common on FWD cars) means the mounting location is towards the outside of the rim. Negative offset (common on deep dish rims) means the mounting location is towards the inside of the rim.
Offset is extremely important to consider when buying rims.

OFFSET
To work out the offset the wheel must be measured accurately using the following steps and using Diagram 1 & Diagram 2.

 
  • Measure the overall width of the wheel - OW *
  • Measure the ‘T’ dimension, which is the measurement from the back of the bolt up face to the outside of the inner rim
  • Divide the overall width by two which gives you the centre line.
  • Minus the ‘T’ dimension from the centre line.

ET = T - OW/2


 

 


tire size is measured with the first number as the width (in mm), the second number is the percentage of the width that is the sidewall height (aka profile), and the third number is the rim size (in inches) that the tire sits on.

your sidewall (the second number) must be taken into consideration depending on what you'll use it for. if you want your rim safe, go with a 50+ sidewall. however, don't go too big or your rim may no longer fit in the wheel well! 40 will normally work for daily driving, but you may run it a bit close in potholes, etc. any lower than a 35 should be used for show or track only! keep those rims safe!

 

Bolt pattern or lug pattern or bolt circle is determined by the number of bolt holes and the bolt circle diameter.

Hub Diameter or center bore is the hole at the center of the wheel.

Rear spacing or back spacing is the distance from the backside of the wheel mounting pad to the outside of the rim flange.

Offset:  The distance from the centerline of the wheel to the mounting surface of the wheel.

Negative offset:  When the back of the bolt pad is closer to the inside of the wheel; when mounting surface is inboard of the rim centerline.

Positive offset:  When the back of the bolt pad is closer to the street side of the wheel; when the mounting surface is outboard of the rim centerline.

 
  Bead-Loc  A device which captures the tire bead between it's flanges, usually secured by bolts to keep tire bead from dismounting.  Usually used in dirt circle track or off road applications where low tire pressures are used and hitting ruts or other vehicles are common.  Left:  An example of a Bead-Loc wheel

 

Determine Wheel Caliper Clearance

  Ensuring proper caliper clearance inside the wheel is important.  The following chart should enable you to have the dimensions required by most wheel manufactures.

 

A. Caliper Overhang Distance
  • Used to determine if wheel dish is adequate (in some cases a spacer is required for clearance)
B.  Diameter of Hub Center
  • Required if wheels are hub centric
C.  Wheel Stud Diameter
  • Required along with bolt circle

D.  Height of Hub Center

E.  Length of Lug and Thread Type (Fine or Coarse)
  • Required to determine if longer studs are necessary
F.  Distance from CL of Hub to Caliper
  • Used with A to determine if a spacer is required for proper fitment
G.  Width of Caliper
  • Used with F to determine if wheel ID is adequate to clear rotor/caliper package
H.  Diameter of Hub Mounting Face
  • Used to determine if hub is adequate to support wheel/spacer