Measuring bolt patterns is a critical part of ordering wheels.  The correct terminology for referencing bolt patterns is to

say the number of lug holes/studs on measurement, such as : 5 on 4.5 or 6 on 135mm.

The distance that you are measuring is the diameter of a circle intersecting the center of each lug hole/stud.

       Measuring Bolt Patterns

All Even-numbered lugs can be determined by simply measuring the distance  from the center of one lug hole/stud to the center of the opposite lug hole/stud across the centerbore.

This applies to 4, 6, 8 or 10 lug vehicles and wheels
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All Odd-numbered lugs can be approximately determined by measuring the distance  from the center of one lug hole/stud to the outside edge of the opposite lug hole/stud across the centerbore.
 
This applies to 3, 5 or 7 lug vehicles and wheels.


 

Many times, aftermarket wheels are drilled with two bolt patterns  on one wheel.  This allows us to fit more vehicles with less inventory.  You will typically see two 4-lug, 5-lug or 6-lug bolt patterns on one wheel.  There are rare cases where you will notice a 4-lug and 5-lug combination on one wheel.  This is achieved by using one lug hole for both bolt patterns as illustrated in the diagram above:


Notice the dual colored lug hole is used for the 4-lug and 5-lug application.  An example would be a 4 x 100 and 5 x 100 bolt pattern.