Tire Size Tutorial

The tire size, intended purpose, dimensions, load capacity and high temperature/high speed durability are imprinted on the sidewall of your tires.  Really, the stuff on the sidewall of your tires says all of this and more.

Let’s look at an example tire that says this on the sidewall:

P225/50R16 91S

“P”225/50R16 91S – The “P,” it signifies the tire is a “P-metric” size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. These include the cars, SUVs & minivans most of us drive today.  Other letters that start the series include:

• T = If the tire is a “Temporary Spare” (“space saver” or “mini spare”)
• LT = If a tire is for a  “Light Truck”
• C = If a tires is used on a “Commercial” vehicle like a delivery truck
• ST = If the tire is a “Special Trailer Service” size that was designed to only be used on boat or utility trailer

Tire Width

P“225”/50R16 91S – Following the letter there is a three-digit numeric portion that identifies the tire’s “Section Width” in millimeters.  In our example, the 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel.

Sidewall Aspect Ratio (tire’s profile)

P225/“50”R16 91S – Typically following the Tire Width there is a two-digit number that identifies the tire’s profile.  In our example, the 50 indicates that this tire size’s sidewall height (from rim to tread) is 50% of its section width.

Internal Construction

P225/50“R”16 91S – a letter that identifies the tire’s internal construction follows the two digits used to identify the tire’s profile.  Again in our example, the letter R means our tire has “radial construction.”   Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.  Letters in this section other than “R” include:

• D=the internal tire body plies crisscross on a Diagonal and that the tire has a “bias ply” construction.
• B= the tire body plies not only crisscross the tire on a diagonal as in the D tire, they are reinforced with belts under the tread area. This type of tire construction is called “Belted.”
• RF=identifies tires with self-supporting run-flat construction. These tires are “radial construction” by design,

Tire and Wheel Diameter

P225/50R“16” 91S – The “16” in your tire indicates the tire and wheel diameter are designed to be matched together. Our example tires go on a 16-inch wheel rim.

Service Description

P225/50R16 “91S” – The 91S represents the tire’s Load Index and Speed Rating. Looking at the 2 charts below, our “91S” example tire is rated for 1356 pounds

Z Speed Rated Tires

P225/50R”Z”16 – All speed ratings for tires are in the tire’s “Service Description” as stated above except for tires with the speed rating of “Z.”  The “Z” rating appears next to the sidewall aspect ratio (tire’s profile).  Prior to 1991 tires had the speed rating in this location next to the sidewall aspect ratio and the letters meant:

B – Up to 31 mph
C – Up to 37 mph
D – Up to 40 mph
E – Up to 43 mph
F – Up to 50 mph
G – Up to 56 mph
J – Up to 62 mph
K – Up to 68 mph
L – Up to 75 mph
M – Up to 81 mph
N – Up to 87 mph
P – Up to 94 mph
Q – Up to 100 mph
R – Up to 106 mph
S – Up to 112 mph
T – Up to 118 mph
U – Up to 124 mph
H – Up to 130 mph
V – Up to 149 mph
W – Up to 168 mph
Y – Up to 186 mph
Z – 149 mph and over

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