I've been working on vehicles for 25 years as an ASE Master Technician, and tire questions come up constantly. People know they need new tires but have no idea what they're looking at when they read the sidewall. And when it comes to tread depth — most drivers wait far too long to replace worn tires. This article covers both: how to read every number on your tire and exactly how to check whether yours have enough tread left to be safe.
Breaking Down the Tire Size Code
Look at the sidewall of your tire and you'll see something like this: 225/55R17 97H
Let's decode each part.
225 — Tire Width in Millimeters
This is the width of the tire from sidewall to sidewall, measured in millimeters. A 225 is 225mm wide, which works out to about 8.9 inches. Wider tires generally provide a larger contact patch and more grip, but they also add rolling resistance and may not clear the fender on every vehicle. The width must match the wheel rim width — you can't mount a very wide tire on a very narrow rim.
55 — Aspect Ratio (Profile)
The number after the slash is the aspect ratio — the height of the sidewall expressed as a percentage of the tire's width. A 55 means the sidewall height is 55% of 225mm, which equals about 124mm (roughly 4.9 inches). Lower aspect ratio numbers mean shorter, lower-profile sidewalls. A 35-series tire is a very low-profile performance tire. A 75-series tire has a taller sidewall, more common on trucks and SUVs. Taller sidewalls absorb more road impact; shorter sidewalls handle better but ride stiffer.
R — Radial Construction
The letter R indicates radial tire construction — the internal steel belts run perpendicular to the direction of travel. Every modern passenger car and light truck tire is radial. You may occasionally see "B" for bias-ply (older construction still used on some trailers and specialty applications) but virtually never on a regular vehicle tire.
17 — Wheel Diameter in Inches
This is the diameter of the wheel rim the tire is designed to fit, in inches. A 17 means the tire fits a 17-inch rim. This must match your wheel exactly — a 17-inch tire will not fit on an 18-inch rim. Note this is the only measurement in inches; everything else is metric.
97 — Load Index
The load index is a number that corresponds to the maximum weight each tire can support when properly inflated. A 97 corresponds to 1,609 pounds per tire. You can find load index charts online or at any tire shop. You should never replace tires with a lower load index than specified — especially on trucks and SUVs that may be used for towing or hauling. A higher load index is always acceptable.
H — Speed Rating
The speed rating is a letter indicating the maximum sustained speed the tire is engineered to handle safely. Common ratings you'll encounter:
- T — 118 mph (common on family sedans and minivans)
- H — 130 mph (very common on passenger cars)
- V — 149 mph (performance cars and sport sedans)
- W — 168 mph (high-performance vehicles)
- Y — 186 mph (exotic/sports cars)
Always replace tires with an equal or higher speed rating. A lower rating means the tire construction is not designed for the heat and stress your vehicle might generate, even in normal highway driving.
Other Markings Worth Knowing
DOT Code and Date — Every tire has a DOT compliance code ending in four digits. Those last four digits are the manufacture date: the first two digits are the week, the last two are the year. "1524" means week 15 of 2024. Tires age even without being used — most manufacturers recommend replacing tires older than 6 to 10 years regardless of tread depth, because rubber hardens and can crack.
Max Load and Max Pressure — Molded separately on the sidewall. This is the absolute maximum, not the recommended inflation for your vehicle. Your vehicle's recommended pressure is on the door jamb sticker. These are different numbers — don't confuse them.
M+S or Three-Peak Mountain Snowflake Symbol — M+S (mud and snow) indicates a general all-season rating. The three-peak mountain snowflake symbol (a mountain outline with a snowflake inside) indicates the tire meets specific performance standards for severe snow conditions. These are different certifications — a tire can have M+S without the snowflake symbol. For more on choosing the right tires for your climate and vehicle type, check out the free automotive guides at APEX Tech Nation.
How to Measure Tire Tread Depth
Tread depth is measured in 32nds of an inch. A new tire typically starts at 10/32 to 11/32 inch. The legal minimum in most U.S. states is 2/32 inch. But legal minimum and safe are two different things.
Method 1: Tread Depth Gauge (Best)
A tread depth gauge is a small tool that slides into the tread groove and gives you a direct reading in 32nds of an inch. They cost under $10 at any auto parts store and take about 10 seconds per tire. Insert the probe into the lowest point of the tread groove in multiple locations across the tire — center and both outer edges. Note the lowest reading; that's your true tread depth at the most worn point.
Method 2: The Quarter Test (Early Warning)
Insert a quarter into the tread groove with Washington's head pointing down into the tire. If you can see the top of Washington's head, you're at approximately 4/32 inch. At 4/32 inch you should start planning to replace the tires — wet weather performance has already started degrading meaningfully.
Method 3: The Penny Test (At Legal Minimum)
Insert a penny with Lincoln's head pointing down. If you can see all of Lincoln's head, you're at 2/32 inch or below — legally worn out in most states and genuinely unsafe in wet or slippery conditions. If you've reached the penny test stage, the tire replacement should have happened a few thousand miles ago.
Method 4: Tread Wear Indicators
Every modern tire has tread wear indicator bars — small raised bars molded into the bottom of the tread grooves at 2/32 inch height. When the tread is worn down to the level of these bars, they become visible as continuous strips running across the tread. When you can see these bars flush with the surrounding tread, the tire is at minimum. This is the simplest visual check available — no tools needed.
What Uneven Tread Wear Is Telling You
Where your tires wear tells you exactly what's wrong with the vehicle:
- Center wear, both edges fine — Tire has been chronically overinflated. Reduce pressure to the door jamb specification.
- Both outer edges worn, center fine — Chronic underinflation. The sidewalls are doing work the tread should be doing.
- One edge worn significantly more — Alignment problem, typically camber. Get the alignment checked.
- Cupping or scalloping — Uneven high and low spots. Almost always worn shock absorbers or struts allowing the tire to bounce.
- Feathering (one side of each tread block rounded, other side sharp) — Toe misalignment. The tire is effectively driving sideways at a slight angle.
Replacing tires without correcting the underlying issue wastes money — the new tires will wear the same way.
The Practical Takeaway
Reading a tire size is a skill that takes about two minutes to learn and serves you forever. Knowing your size means you can shop for tires confidently, verify quotes from shops, and catch it immediately if someone tries to install the wrong tire. And checking tread depth twice a year — at the same time as your oil change is an easy habit — keeps you ahead of the wear before it becomes a safety issue rather than catching it after the fact.