Offset is very important in determining if a set of rims will fit on a car. In addition to the width, this number helps to show how flushed to the fender the rim sits. This is not the definition of offset (but you can find that on the net ;)) but in the real world, the combination of those two numbers will determine how the rim sits in your fender well.
The lower the number, the farther out the wheel moves towards the fender. For example, 20x8.5 +38 will be inside the fender while 20x8.5 +10 will sit outside the fender. (The offset number is a mm neasurement.)
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i learned something new today thanks