According to TireSize.com, the tire sizes for a 1981 Ford F150 range in width from 215 to 235 millimeters. The aspect ratio is 75 and the diameter is 15 inches. The tire size depends on the drive type.
The tire sizes are 215/75R15 or 235/75R15 for a 1981 Ford F150 two-wheel-drive pickup truck and 235/75R15 for a four-wheel-drive truck. The diameter of a 215/75R15 is 27.7 inches with a circumference of 87 inches and turns at 728 revolutions per mile. The diameter of a 235/75R15 is 28.9 inches with a circumference of 90.7 inches and turns at 699 revolutions per mile. Tire size specifications vary by manufacturer.