Tire Sizing, Wear, and Rotation
Read a tire's size, load, speed and date codes, judge wear patterns and tread depth, and rotate tires in the right pattern.
Table of contents
A tire tells you a lot before you touch a tool. The sidewall gives its size, what it can carry, how fast it can run and how old it is. The tread shows how the vehicle has been treated and what may be wrong with it. Reading both correctly keeps you from fitting the wrong tire, missing a safety problem or sending a customer away with a car that will eat its next set.
What You Will Be Able to Do
- Decode a tire size, load index, speed rating and DOT date code.
- Choose a replacement that meets the vehicle's requirements.
- Measure tread depth and explain what the numbers mean.
- Read a wear pattern and name its likely cause.
- Rotate tires in the correct pattern for the drivetrain and tire type.
Reading the Sidewall
Take a tire marked 225/60R17 98V.
| Code | Meaning |
|---|---|
| 225 | Section width in millimeters |
| 60 | Aspect ratio: sidewall height is 60 percent of the width |
| R | Radial construction |
| 17 | Wheel diameter in inches |
| 98 | Load index: 1,653 pounds per tire |
| V | Speed rating: 149 mph |
A letter in front of the size names the tire's service type. P means passenger, LT means light truck and ST means special trailer. A size with no letter is usually a metric tire, also called Euro metric. An ST tire is for trailers only, and an LT tire on a pickup has a load range letter such as E that matters as much as the size.
Worked Example: Sidewall Height and Overall Diameter
For 225/60R17, the sidewall height is 225 x 0.60 = 135 mm. The wheel is 17 inches, which is 17 x 25.4 = 431.8 mm. Overall diameter is the wheel plus two sidewalls: 431.8 + 135 + 135 = 701.8 mm, or about 27.6 inches.
This matters when a customer wants a different size. A change in overall diameter changes speedometer readings and can upset stability and ABS systems, and it can rub. Stay with sizes the vehicle maker approves, and treat any other size as a question for your tire lead.
Load Index and Speed Rating
The load index is the most weight one tire can carry at its maximum rated pressure.
| Index | Pounds per tire |
|---|---|
| 85 | 1,135 |
| 91 | 1,356 |
| 94 | 1,477 |
| 98 | 1,653 |
| 102 | 1,874 |
| 105 | 2,039 |
| 109 | 2,271 |
| 113 | 2,535 |
The speed rating is the highest speed the tire is rated to sustain under its rated load.
| Symbol | Speed | Often seen on |
|---|---|---|
| S | 112 mph | Family cars, SUVs |
| T | 118 mph | Family cars, minivans |
| H | 130 mph | Sport sedans |
| V | 149 mph | Performance cars |
| W | 168 mph | High performance |
| Y | 186 mph | Very high performance |
Choosing a Replacement
Start with the placard on the driver's door jamb, not the tires that are on the car. A previous owner may have fitted the wrong size.
- Match the original size, or a size the vehicle maker lists.
- Match or exceed the original load index. Never go below it.
- Match or exceed the original speed rating. Your shop may have a written policy on this.
- Keep the same size and construction across an axle.
- If only two new tires are fitted, the tire makers' usual advice is to put them on the rear axle, so the car keeps rear grip in the wet and is less likely to spin.
The maximum pressure molded on the sidewall is a limit for the tire, not the pressure for the vehicle. Set tires to the pressure on the door placard.
The DOT Date Code
Every tire sold in the US has a DOT code. The last four digits give the week and year it was made. A code ending in 2423 means the 24th week of 2023. A code ending in 2321 means the 23rd week of 2021.
There is no federal age limit for passenger tires. Rubber does age, though, even on a tire that has hardly been driven. Heat and sunlight speed that up.
- Many vehicle makers advise replacing tires 6 years after manufacture, whatever the tread looks like. This is the conservative figure shops use to flag a tire.
- Some tire makers set 10 years as the outer limit for any tire, including the spare.
- The owner's manual and the tire maker's own guidance win when they say something different.
Check the spare and any trailer tires as well. They often look new and are often the oldest tires on the vehicle. Cracks in the sidewall, called weather checking, are a reason to look harder at any tire of any age.
Measuring Tread Depth
Tread depth is measured in 32nds of an inch with a gauge, not a coin. Measure the inner, center and outer grooves on each tire, and check more than one spot around the tire.
| Depth | What it means |
|---|---|
| 10/32 to 11/32 in | About new for most all-season tires |
| 6/32 in | Worn, still serviceable |
| 4/32 in | Noticeably less wet grip, start the conversation |
| 3/32 in | Recommend replacement |
| 2/32 in | Legal minimum in most states, tread wear bars are flush |
Most states set 2/32 inch as the legal minimum, but states do vary, so check yours. The small raised bars molded across the grooves are wear indicators. When the tread is level with those bars, the tire is at 2/32 inch.
Keep tire depths close across an axle. All-wheel-drive vehicles are stricter, because a tire with a smaller diameter turns faster and strains the center differential or transfer case. Many makers want all four tires within 2/32 inch of each other. Check the owner's manual before replacing just one or two.
Reading Wear Patterns
Before you sell a tire, ask why the old one wore out. A new tire on an unrepaired problem wears the same way.
| Pattern | Likely cause | What to do |
|---|---|---|
| Both outer edges worn | Under-inflation | Set pressure, look for a slow leak |
| Center worn | Over-inflation | Set pressure to the placard |
| Inner edge worn | Too much negative camber, or toe-out | Inspect the suspension, check alignment |
| Outer edge worn | Too much positive camber, or toe-in | Inspect the suspension, check alignment |
| Feathered, like a saw tooth | Toe out of spec | Check alignment |
| Cupping or scallops | Worn shocks or struts, or imbalance | Inspect dampers, balance the tire |
| Flat spot | Locked brake or long storage | Inspect brakes, replace if it vibrates |
| Bulge in the sidewall | Broken internal cords from an impact | Replace now, do not drive on it |
To feel for feathering, slide your palm across the tread in one direction and then the other. A feathered tire feels smooth one way and sharp the other. Feathered and cupped tires are often loud, and customers may report the noise as a bad wheel bearing.
Warning: A sidewall bulge, exposed cords or a tread separation means the tire can fail suddenly. Mark the tire as unsafe and tell the service advisor right away.
Rotating Tires
Tires wear differently at each corner. Drive wheels work harder, and front tires on a front-wheel-drive car carry the steering, most of the braking and the drive load. Rotation evens this out so that the set wears together. Follow the owner's manual for the interval, which is often about every 5,000 to 8,000 miles.
| Vehicle or tire | Pattern | How it works |
|---|---|---|
| Front-wheel drive | Forward cross | Fronts go straight back, rears cross to the front |
| Rear-wheel drive or 4WD | Rearward cross | Rears go straight forward, fronts cross to the rear |
| All-wheel drive | Rearward cross or X, per the manual | Check the vehicle maker's chart |
| Directional tires | Front to rear, same side | Tires stay on their side |
| Staggered, non-directional | Side to side, same axle | Front and rear sizes differ |
| Staggered and directional | No rotation without remounting | Usually left in place |
A directional tire has an arrow on the sidewall showing which way it turns. Crossing one to the other side would make it run backward and hurt its wet grip.
After a rotation:
- Check every tire's pressure and set it to the placard. Some vehicles list different front and rear pressures.
- Torque the wheels to spec with a torque wrench.
- Check whether the vehicle's tire pressure system needs a relearn, since some systems show pressure by wheel position.
Rotation is also your chance to catch problems. With the wheel off, you can see brakes, suspension parts and the inner sidewall. Note what you find.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Sizing from the tires on the car, not the placard | Wrong tire, and possibly an overloaded one |
| Fitting a lower load index | Overheating and tire failure under load |
| Setting pressure to the sidewall maximum | Harsh ride, center wear, less grip |
| Skipping the spare's date code | A failed spare when the customer needs it |
| Crossing directional tires | Poor wet grip, noise, an unhappy customer |
| Replacing one tire on an AWD car without measuring | Driveline damage |
Check Your Understanding
- A customer's tire reads 215/55R18 95H, and the shop has a matching size rated 91V in stock. Is it a good substitute? No. V beats H on speed, but 91 is a lower load index than 95. Find a tire that meets both.
- A tire with deep, even tread has a DOT code ending in 1216, and today is late 2026. What do you tell the advisor? It was made in the 12th week of 2016, so it is over 10 years old. Flag it for replacement whatever the tread depth.
- A set of front tires is worn on both outer edges, with the center still good. What is your first check? Tire pressure and any slow leak. Wear on both edges points to under-inflation.
- A front-wheel-drive car has non-directional tires. Where does the left rear tire go? To the right front. Rears cross forward and fronts go straight back.
- An AWD customer wants one new tire. The other three measure 5/32 inch and the new one is 10/32 inch. What is the issue? The difference is more than most makers allow. Check the manual, since the vehicle may need a matched set or a tire shaved to match.
Key Takeaways
- The door placard, not the tires on the car, tells you what the vehicle needs.
- Match or exceed the load index and the speed rating.
- The last four DOT digits are the week and year. Flag tires at 6 years and treat 10 years as the outer limit, unless the maker says otherwise.
- Measure tread in three places, and keep depths close on AWD vehicles.
- A wear pattern is a symptom. Fix the cause before you fit new tires.
- Use the rotation pattern that suits the drivetrain and the tire type.