Measuring 4-Bolt Trailer Wheels
Measure a four bolt wheel from the center of a bolt hole, skip one bolt hole and then the center of the next hole. Basically your ruler goes across the center of the wheel.
Measuring 5-Bolt Trailer Wheels
Measure a five bolt wheel from the edge of one bolt hole, skip one bolt hole and then the center of the next hole. In this case the ruler is not across the center of the wheel and you measure from the edge of one to the center of the other.
Measuring 6-Bolt Trailer Wheels
Measure a six bolt wheel from the center of the bolt hole, skip 2 bolt holes and then the center of the next hole. Your ruler goes across the center of the wheel.
Measuring 8-Bolt Trailer Wheels
Measure an eight bolt wheel from the center of a bolt hole, skip 3 bolt holes and then the center of the next hole. Your ruler goes across the center of the wheel.