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Usually the easiest way to find out is to look for where the wheel size is cast into it somewhere. It'll say something like 7HJ15 ET48.5. The 7 is the width in inches. The 15 is the diameter in inches. And the "ET" number is the offset in millimetres.
If you can't find it you need to measure it. You need a metric tape measure (or a calculator and the knowledge that 1"=25.4 mm) and two straight pieces of metal or wood at least 18" or 500 mm long. With the wheel removed from the car you put one straight piece diagonally across the outer side of the wide, and the other across the inner side, then use the tape measure to measure two things. The distance between them, and the distance from the inner straight piece to the machined surface on the wheel that bolts up to the hub.
Say the distance between the two straight pieces is 200 mm, you divide it by two to find the centreline of the wheel, which in this case would be at 100 mm. Then if the distance from the inner straight piece to the where the hub bolts to the wheel is 138 mm your offset is +38 mm.
That's what the offset is. How far out from the centreline of the wheel the surface that bolts up to the hub is.
If you can't find it you need to measure it. You need a metric tape measure (or a calculator and the knowledge that 1"=25.4 mm) and two straight pieces of metal or wood at least 18" or 500 mm long. With the wheel removed from the car you put one straight piece diagonally across the outer side of the wide, and the other across the inner side, then use the tape measure to measure two things. The distance between them, and the distance from the inner straight piece to the machined surface on the wheel that bolts up to the hub.
Say the distance between the two straight pieces is 200 mm, you divide it by two to find the centreline of the wheel, which in this case would be at 100 mm. Then if the distance from the inner straight piece to the where the hub bolts to the wheel is 138 mm your offset is +38 mm.
That's what the offset is. How far out from the centreline of the wheel the surface that bolts up to the hub is.
