When you go to buy new alloys for your new Civic or Corolla, there is a trend of installing alloys that are slightly wider than the original. Or sometimes, you end up buying alloys that are wider than you were expecting them to be and you end up rubbing the tires under the fenders of the car. By looking at the bolt pattern, you thought they look fine, but when you install them, you find they are slightly more outside than you like them to be.
The thing that tells you how far in or out is the bolt holes area of the wheel is called ‘offset’ of the wheel. It is denoted by ET.
Image a straight line right through the center of the wheel. Offset is measured according to that line. So lets say a wheel is 6 inch wide. Imaginary line would be at 3 inches deep, right in the middle of the wheel. Now how far are the mounting holes from that 3 inch center would tell you the ET of the wheel.
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You can find the wheels both with positive and negative offset. Most modern cars come will alloys that have positive offset, which means the mounting face is forward of the imaginary center line. Older cars and usually 4x4s have wheels with zero or negative offset. Zero as in the plane of the hub mounting surface is right in the middle of that center line, and negative if the mounting surface is inward to the wheel.
Positive offset alloys will be more towards the inwards of the vehicles, whereas negative offset meanings they will be more towards the outwards of the car.