Front-wheel setback is measured through the spindles perpendicular to the vehicle's centerline. Front-wheel setback causes a vehicle to drift to the side with the most setback.
A rear-wheel setback is when one of the rear wheels sits further back than the other. It causes the vehicle to drift to the opposite side. Look for a bent component or a broken leaf spring or center bolt.
Check the vehicle manufacturer's specifications for a built-in setback to compensate for road design and crowning. Designed for water drainage, the center or dome of a crowned road is higher than its shoulders. A vehicle with an even setback drifts towards the shoulder of the road. Manufacturers compensate for this drift by setting the wheel closer to the crown of the road back, slightly further than the other.