Lead/Pull is the deviation from a straight path on a level road with no pressure on the steering wheel. The following are common causes of Lead/Pull:

The way in which a tire is built can produce lead. An example if the placement of the belt. An off center belt on radial tires causes the tire to develop a side force while rolling straight down the road. The tire tends to roll like a cone. Refer to Vehicle Leads/Pulls in General Suspension Diagnosis to make sure that the wheel alignment is not mistaken for tire lead. Rear tires usually do not cause lead/pull.