Tire Terms, Dimensions, and Definitions
Free radius (2) is the radius of the mounted wheel and tire assembly when it is properly inflated and not deformed by any load. It is measured from the axle centerline to the road-contact surface of the tread.
Loaded section height (3) is the static loaded radius minus half of the rim diameter. This is equal to the distance from the road surface to the rim seat.
Nominal wheel diameter (A) is the distance between the bead seats, as measured at the widest point. It may be expressed in inches or millimeters.
Overall diameter (4) is the maximum distance across a mounted and properly inflated tire that is not under load. It is measured from the road-contacting surface of the tread on one side of the tire to the road-contacting surface on the opposite side. It is usually expressed in inches, but may be expressed in millimeters.
Overall width (5) is a tire’s maximum width from sidewall to sidewall, and includes all protruding elements, such as lettering and protective ribs.
Revolutions per mile is the number of revolutions a loaded tire makes in one mile. It can change with load, wheel width, inflation and speed.
Rim width is the distance between the inner and outer bead-seat flanges.
Rolling circumference is straight-line distance traveled by a tire during one full revolution. It can change with load, inflation and speed.
Section height (6) is the distance from the bead seat to theoutermost surface of the tread of a mounted tire not under load.
Section width (7) is the width of a tire from sidewall to sidewall. It does not include any protruding elements, such as lettering or protective ribs.
Static loaded radius (8) (also called loaded radius) is the distance from the centerline of the axle to the road. It refers to a properly mounted and inflated tire under a prescribed load.
Tread depth is the distance to the bottom of the tread grooves. It is usually expressed in 1/32 inch increments.
Tread width (9) is the distance straight across a tire’s tread, from the inner edge to the outer edge of the road-contact surface.