Unravelling the mystery of tire sidewall numbers, letters
The sidewall of a tire has a lot to tell you — if you know how to read it. Let’s take, for example one that says 215/55/R/17 98H.
215: This number tells us the width or cross section of the tire as measured in millimetres from sidewall to sidewall. Obviously, the larger this number the wider the tire. Some high-end sports cars have tires up to 335 mm wide on the rear wheels.
55: This is known as the aspect ratio or height of the sidewall from rim to road expressed as a percentage of the section width — in our example 55 per cent of 215 mm. This is a fairly common number today. Older car tires had aspect ratios of 70 and even 80.
As this number goes down, so does the amount of rubber visible from the side of the tire. Those ultra-wide sports car tires mentioned above could have an aspect ratio as low as 30 and be designated as 335/30. This number can comes in increments of five i.e. 60, 55, 50 etcetera.
R — Radial construction: Most common passenger vehicle tires are of radial construction today. There are very few bias belted tires available — or wanted.
17 — Wheel diameter in inches: The size of the hole in the centre of the tire. Don’t ask why this measurement is universally given in inches, while the remainder are metric.
98 — The load rating or index: This number refers to a chart used in the tire industry and indicates the maximum amount of weight this particular tire is designed to carry. The numbers on the chart range from 71 to 111. Your tire dealer will have a copy of this chart showing the amount in pounds or kilograms.
Our example of 98 means a load rating of 1,653 lbs. per tire, or a total of 6,612 pounds. A rating of 111 corresponds to 2,403 pounds meaning a vehicle so-equipped could carry 9,612 pounds.
These weights include vehicle, passengers, luggage and everything else. Finding a tire with a higher load rating than others of the same size is important when carrying heavy loads or towing.
H— Speed rating: This number comes from another standard industry rating system and resultant chart. The letter indicates the maximum sustained speed the tire is designed to cope with. The letters range from H to Z but not necessarily in order. The letters are their corresponding speed rating in km/h are: Q-160, S-180, T-190, U-200, H-210 (our sample), V-240, W-270, Y-300 and Z-over 300.
In addition to the information outlined above most tires also comply with UTQS — uniform tire quality standard — that provides consumers with information on three categories — tread wear, traction and temperature.
Each tire maker conducts its own tests, following prescribed procedures and the information is for comparison only, not a safety rating and not a guarantee of how long the tire will last. The information is located on the tire sidewall and on a sticker attached to the tire when new.
Tread wear: The numbers typically range from 50 to more than 600 indicating how long the tire is expected to last under controlled conditions. For example, a tire with rating of 400 should last twice as long as one carrying a 200 rating. It is important to note each manufacturer uses different rating system, so do not compare a 200 from one with a 200 from another.
Traction: These numbers indicate a tire’s ability to stop on wet pavement, under controlled test conditions on asphalt.
Twenty measurements are taken on asphalt and 20 more on concrete. The ratings range from highest or best to lowest, AA through A, B and C.
Temperature: These ratings indicate a well-maintained tire’s ability to resist and disperse heat. Tires are tested on a rolling indoor test machine starting at 121 km/h and increased in eight km/h increments until the tire fails. The ratings are based on a tire that is properly inflated and not overloaded — the chief causes of tire failure. The ratings range from highest (A) to lowest (C) – the minimum required by federal standards.
Other information contained on a sidewall includes:
Maximum pressure: The maximum recommended tire pressure.
Design usage: M+S or MS or M&S indicates an all-season tire. A little peaked mountain with a snowflake indicates the tire has met specific snow traction performance requirements and is designed specifically for use in severe winter conditions. A tire with neither of these markings is designated for summer use only.
Tires also carry an approved code that identifies the manufacturer, plant, tire make and model, date of production.
The next time you think of curling up with a good book, consider curling up on the garage floor and reading a tire.