Tire Terminology

From the Chapman Report - February 1991 & other sources
By Kiyoshi Hamai

Tire makers around the world are standardizing the ratings and the way they designate the sizes of tires. Typically a tire will be marked... 205/60-13. The 205 is the width in millimeters and the 60 refers to the aspect ratio, or the height of the sidewall as compared to the width (sidewall to sidewall) expressed in terms of a percentage. Thus 60 would mean a 60% aspect ratio or the height of the sidewall is 60% of the width or .60 x 205 or 123mm. The 13 is in inches and refers to the diameter of the wheel used.

Now comes some new numbers and letters... Here's an example...

P205/60R13 86H

The "P" is the U.S. means of designating a "P" metric tire size. The 205/60 is the same as above. The "R" refers to the fact that the tire is a RADIAL. The 13 is again the wheel diameter. But, then comes the 86H... The 86 is the load rating designation. It is a code to designate the maximum load that the tire can sustain at the speed indicated by the Speed Symbol, which is the "H" in this case. Load indexes for passenger car tires typically range from 75 ot 100. The Speed Rating is the "H" and refers to the tire's maximum sustained speed rating (see table).

Letter / Maximum RatingSpeed (kph/mph)
L = 120/ 74.5
M = 130/ 80.7
N = 140/ 86.9
P = 150/ 93.2
Q = 160/ 99.4
R = 170/105.6
S = 180/111.8
T = 190/118.1
H = 210/130.4
V = 240/149.1
Z = 240+/149.1+
W = maximum speed 168 mph for exotic sports cars
Y = maximum speed 186 mph for exotic sports cars

The above Z rating has been expanded and is still used but often in conjunctino with W or Y.

Performance Tire Segments

Summer Passenger Performance Segment
Extreme Performance: For drivers who want extreme dry street performance and are willing to trade some comfort and hydroplaning resistance to get it.
Maximum Performance: For drivers who want an unsurpassed blend of dry and wet street traction and handling and only the finest will do.
Ultra-high Performance: For drivers who want an outstanding blend of dry and wet street traction and handling.
High Performance: For drivers who want a good blend of dry and wet street performance when "re-tiring" their sports coupes or sedans.
Grand Touring: For drivers who want a good blend of dry and wet street performance along with responsive handling and good noise and ride comfort.
Track and Competition DOT: For drivers who want DOT legal street tires for autocross, racetrack and rally performance.

All-Season Passenger Performance Segments
Ultra-high Performance: For drivers who want all-season versatility (including light snow traction) and are willing to trade some dry and wet traction and handling to get it.
High Performance: For drivers who want all-season versatility (including light snow traction) to drive their sports coupes or sedans in all weather conditions.
Performance: For drivers who want all-season versatility (including light snow traction) with a mixture of performance and appearance at a reasonable price.
Grand Touring: For drivers who want all-season versatility (including light snow traction) and responsive handling along with noise and ride comfort.
Standard Touring: For drivers who want all-season versatility (including light snow traction) along with enhanced handling and tread wear, as well as good noise and ride comfort.
Performance Winter/Snow: For drivers who want enhanced dry road handling from their winter/snow tires and are willing to trade some snow and ice traction to get it.

Tire Terminology


Overall Diameter: th outside diameter of the tire when mounted and inflated but with no vehicle load.
Section Width (also call overall width): the maxiumum width of the tire's cross section of an unloaded, mounted and inflated tire (the widest point of the tire when mounted and inflated, but with no vehicle weight).
Free Radius: the radius of the tire/wheel assembly that isn't affected by load. This is the distance from the wheel axle centerline to the top of the tire tread face.
Loaded Radius: the distance from the wheel axle centerline to the ground, drawn vertically. This is the distance from the vehicle hub centerline to the ground when a tire is inflated and when the tire supports vehicle weight.
Nominal Wheel Diameter: This refers to the size of the wheel applicable for mounting the tire (diameter of the rim seat that will support the tire bead). The bead-to-bead diameter is measured at the inside diamter of the tire, once mounted.
Section Height: the distance from the bead to the tread face.
Loaded Section Height: the loaded radius minus half of the nominal rim diamter.
Aspect Ratio: This referes to the tire's section height in relation to its section width, as a percentage. For example, a 60-series tire features a sidewall that is 60% as tall as the tire's section width. A 50-series tire will feature a shorter sidewall, approximately 50% of the section width.
Tread Width: This is the distance measured from the inner tread shoulder to the outer tread shoulder. Tread width is not to be confused with section width, which is always greater than the tread width.
Plus-Zero: This maintains the original wheel diamter, but increases tire section width by 10mm, and drops the aspect ratio by 5%. For example, if the vehicle was originally equipped with 205/55R15 tires, you could keep the same wheels and move to a 215/50R15 tire. This provides a slight increase in tread contact patch, along with a slightly shorter sidewall for quicker steering response and less lateral lean, while maintaining the original overall tire diameter.