Anything above 2" is unsafe, in general I would never use wheel spacers at all. Larger than 2" allows for the possibility of the wheel flexing away from the axle especially if the lug nuts are not tightened the the correct torque rating and if they are over tightened it could cause to much stress on the studs, causing them to snap or break at the most inconvenient time, like driving down the freeway.
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