The linear sum toe-in is when the distance between the front wheels is less at the front of the axle than at the rear of the axle.

The angular toe-in is the angle of the front wheel centerlines (1), relative to the intersection of the forward projected vehicle center line (2).

The actual amount of toe-in is normally a fraction of a degree. The toe-in is measured from the center of the tire treads (2) or from the inboard side of the tires. The purpose of toe-in is in order to ensure parallel rolling of the front wheels and the offset any small deflections of the wheel support system that occurs when the vehicle is rolling forward. The excessive tire wear and unstable steering are a result of incorrect toe-in. The toe-in is the last angle to be set in the front wheel alignment procedure. The toe-in is the most critical factor related to the tire wear.