The wheel center ring is the ring that goes inside the insert (THIS IS USUALLY A LARGER DIAMETER THAN THE OUTER RIM DIAMETER) For Example, 1.0 beadlock wheels generally have a center ring diameter between 27mm-30mm and 1.0 PLUS beadlock wheels generally have a center ring diameter between 30mm-33mm. (There may be some exceptions where some 1.0 wheels have center rings smaller than 27mm, or larger than 30mm, but that is uncommon.) You will have to dismount your tire from the wheel to get the accurate center ring diameter, alternatively you can look up your wheels online, as dimensions are normally readily available for most wheel types. (SEE PICS BELOW)