This is quoted from the pages of etrailer.
Determining Wheel Offset
Wheel offset refers to the location of a wheel's rim in relation to the wheel's center. The rim can be centered on the wheel or it can be offset to either side. The position of the rim determines where the tire lines up - either closer to or farther from the vehicle or trailer on which it is mounted. Trailer wheels usually have zero offset, meaning that the rim of a trailer wheel lines up directly under the center of the wheel. This alignment puts the tire directly under the center of the wheel, giving the wheel and tire combination the greatest ability to handle load.
To determine if your trailer wheels have an offset, remove a wheel from your trailer and measure from the mounting surface of the wheel (where the wheel mounts to the hub) to the inside and outside beads. The difference between these two measurements is the offset.
Because some trailer wheel offsets can be as small as 5 mm, they can be difficult to measure. If you can't measure the wheel, contact your trailer manufacturer with your trailer's serial number.
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