For 1990-2009 cars

Lead is the deviation of the vehicle from a straight path on a level road, without hand pressure on the steering wheel. Lead is usually the result of one of the following conditions:

1. Tire construction. Refer to Vibration Diagnosis and Correction in Suspension General Diagnosis. Refer also to General Description in Tires and Wheels.
2. Uneven parking brake adjustment. Refer to Park Brake Cable Service/Adjustment in Brakes.
3. Wheel Alignment

The way in which a tire is built may produce lead. Rear tires do not cause lead.