SidewallThis is the part of the tire between the tread and the bead. Tire information (service description) is written on the sidewall.
Wheel DiameterThe wheel diameter shows the diameter of the wheel the tire is designed to fit. It is a measurement between the bottom of the bead seat directly across from each other, using the same side of the wheel.
Tread DepthA tire's tread depth refers to the amount of usable tread that is on the tire. To find out the tread depth of your tires, measure the tread groove in the tire centerline from the the groove base to the top of the tread. To get the best measurement, use a tread depth gauge.
To get a quick measurement of your tire's tread depth, you can use a quarter and a penny. Place a quarter in the tread groove with the head facing down and towards you. If you can see the top of George Washington's head, your tires are worn below 4/32nds of an inch. At this depth, your tires are more prone to hydroplaning and reduces your stopping ability greatly.
With a penny, place the penny with the head facing down and towards you. If you can see the top of Abraham Lincoln's head, your tires are worn below 2/32nds of an inch. A lot of states classify a tire legally worn down at 2/32nds of an inch.