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UK Home of the Opel Manta.
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Wheel Offset ET Explained
The ET (inpressed depth) or offset of a wheel is...
The measured distance From the rear mounting face of the wheel which sits against your hub To the dead center line of the wheel.
Thats it, that measurement is your offset.
If you have a zero offset wheel, the rear of the mounting face (where your wheel bolts pop through the back) will be in the center of the wheel width.
If you have a -(minus) offset,the wheel will no doubt have a bigger dish and will sit more proud of the wheel arch. If you have too high an offset then the wheel will sit further in, possible clearance problems with suspension/brakes/rear beams.
You are best off trying to stick as near the factory offset as possible,running a very low offset will put more strain on the wheel bearings and handle differently (obviously width/tyre choice/wheel diameter and width all come into play too)