Your tires are the only part of the car that touches the road, so a few minutes a month on pressure and tread does more for safety than almost any other habit. Set pressure to the placard on your driver's door jamb, replace tires before the tread gets close to 2/32", and replace them by age even if they still look fine.
This guide shows you how to decode the numbers on the sidewall, measure tread, choose between tire types and avoid the most common mistakes when changing tire sizes.
How to read a tire sidewall
Most passenger tires carry a size code that looks like 225/65R17 102H. Here is what each part means:
| Code | Meaning | In this example |
|---|---|---|
| 225 | Section width in millimeters, sidewall to sidewall | 225 mm, about 8.9 inches |
| 65 | Aspect ratio: sidewall height as a percentage of width | Sidewall is 65% of 225 mm, about 146 mm |
| R | Construction type | Radial, which nearly all modern car tires are |
| 17 | Wheel (rim) diameter in inches | Fits a 17-inch wheel |
| 102 | Load index: the maximum weight each tire can carry | 850 kg, about 1,874 lb per tire |
| H | Speed rating: the maximum sustained speed the tire is rated for | 130 mph (210 km/h) |
Common speed ratings run from S (112 mph) and T (118 mph) through H (130 mph), V (149 mph) and higher for performance tires. When you replace tires, match or exceed the load index and speed rating your vehicle maker specifies. A lower load index is never a safe shortcut, especially on SUVs and pickups.
Some tires start with a letter: "P" means a P-metric passenger tire, and "LT" means light truck, built for heavier loads and usually run at higher pressures.
Finding the DOT date code
Every tire sold in the US carries a DOT Tire Identification Number. The last four digits are the build date: the first two are the week and the last two are the year. A code ending in 2324 means the 23rd week of 2024. It may only be printed on one side of the tire, so check the inner sidewall if you can't find it. Look at the date on new tires before they're mounted, since tires can sit in a warehouse for months.
Tread depth: when worn means unsafe
New passenger tires usually start with about 10/32" to 11/32" of tread. Most US states set the legal minimum at 2/32", and molded wear bars running across the tread grooves become flush with the surface at that depth. Rules vary by state and country (the UK, for example, uses 1.6 mm), so check your local law.
Legal and safe aren't the same thing. Tread grooves channel water away from the contact patch, and as they get shallow, wet-road stopping distances grow and the risk of hydroplaning rises. For drivers who see a lot of rain or any snow, replacing tires at around 4/32" is a sensible target.
Two quick coin tests
- Penny test: Insert a penny into a groove with Lincoln's head upside down. If you can see the top of his head, you're at or below 2/32" and need tires now.
- Quarter test: Do the same with a quarter. If you can see the top of Washington's head, you have less than about 4/32" and should start shopping.
Check several spots across each tire. Wear on only the inner or outer edge points to an alignment problem, and wear in the center often means overinflation. An inexpensive tread-depth gauge gives you an actual number and takes the guesswork out.
Tire pressure: use the placard, not the sidewall
The correct pressure for your car is on the tire and loading information placard, usually inside the driver's door jamb, and in your owner's manual. The number molded into the tire sidewall is the maximum the tire can hold, not what your car needs.
- Check pressure at least monthly and before long trips, with the tires cold, meaning the car has been parked for a few hours.
- Expect pressure to drop roughly 1 psi for every 10°F fall in temperature, which is why warning lights come on during the first cold snap.
- Don't rely on the dashboard warning alone. Most tire pressure monitoring systems only alert you when a tire is already well below the recommended level.
- Check the spare too, if you have one.
Underinflation is the bigger danger: it builds heat, wears the shoulders and can lead to sudden failure at highway speeds. It also costs you fuel. NHTSA's tire safety pages cover pressure, tread and tire recalls in more detail.
Key takeaway: Set pressure cold, to the door-jamb placard, once a month. Replace tires at 2/32" at the latest, or around 4/32" if you drive in rain or snow, and don't run tires past the age limit your tire or vehicle maker recommends.
Rotation and alignment
Rotation moves each tire to a different position so they wear evenly. Front tires on most cars wear faster because they handle steering and most of the braking. A typical interval is every 5,000 to 7,500 miles, which lines up neatly with many oil-change schedules. Your owner's manual specifies the pattern, and some directional or staggered setups can only be swapped side to side or front to back. Our maintenance planner can remind you when the next rotation is due.
Alignment sets the angles of the wheels relative to the car and each other. Have it checked if the car pulls to one side, the steering wheel sits off-center when you drive straight, you see uneven wear, or after a hard pothole or curb strike. A yearly check is reasonable, and it's worth doing whenever you buy a new set of tires.
Choosing a tire type
| Type | Best for | Trade-offs |
|---|---|---|
| All-season | Year-round use in mild to moderate climates | Quiet and long-lasting, but compromised in deep snow, ice and real cold |
| Winter | Regular snow, ice and temperatures below about 45°F (7°C) | Much better grip in cold; wears faster and handles less crisply in summer heat |
| All-terrain | Trucks and SUVs that mix pavement with gravel, dirt and light trails | More road noise and a little lower fuel economy than highway tires |
| Mud-terrain | Serious off-road use in mud, rocks and loose ground | Loud, heavier, shorter tread life and weaker wet-pavement grip |
Look for the three-peak mountain snowflake symbol if you need real winter capability. It means the tire passed a standardized snow traction test. The older "M+S" (mud and snow) marking is based on tread design alone and doesn't guarantee snow performance. Some regions require winter tires by law: Quebec, for example, mandates them from December 1 to March 15. Check your state or province rules, and see our winter driving guide for more.
When to replace tires by age
Rubber breaks down over time, even on a car that sits. Aging is hard to judge by eye, though fine cracks in the sidewall or between tread blocks are a warning sign. Many tire makers recommend a professional inspection each year once tires reach about five years old, and many vehicle and tire makers recommend replacement somewhere between six and ten years regardless of tread. Low-mileage cars, trailers, RVs and spare tires are where old tires most often go unnoticed.
- Read the DOT date code on every tire, including the spare
- Look for sidewall cracking, bulges or cuts
- Measure tread depth at the inner, center and outer grooves
- Note any vibration, pulling or new noise at highway speed
- Run your tires through NHTSA's recall search if you're unsure
Changing tire size and your speedometer
Plus-sizing means fitting larger wheels with lower-profile tires, or simply taller tires. Your speedometer and odometer are calibrated to the original tire's overall diameter, so changing it changes their accuracy.
A 225/65R17 tire is about 28.5 inches tall. Swap to a 245/65R17 and the diameter grows to about 29.5 inches, roughly 3.6% larger. When your speedometer shows 60 mph, you're actually doing about 62 mph, and your odometer under-counts miles by the same margin. Moving to 235/55R18 instead keeps the diameter within about 1%, which is why it's a common plus-one choice.
A general rule is to stay within about 3% of the original diameter and keep the load index at or above stock. Bigger changes can also affect stability control, ABS, clearance and fuel economy. Our tire size calculator compares any two sizes and shows the speedometer difference before you buy.
The bottom line
Learn your size code, check pressure monthly against the door-jamb placard, and replace tires on tread depth or age, whichever comes first. Rotate them regularly, get the alignment checked when something feels off, and match the tire type to your actual climate and roads. If you change sizes, run the numbers first so your speedometer stays honest.
Frequently asked questions
Should I inflate my tires to the pressure on the sidewall?
No. The number on the sidewall is the maximum pressure the tire can safely hold, not the pressure your car needs. Use the recommended cold pressure on the placard inside the driver's door jamb or in your owner's manual.
How long do tires last?
Tread life varies widely, from around 25,000 miles for soft performance tires to 60,000 miles or more for touring all-seasons. Separately, rubber ages, so many vehicle and tire makers recommend replacing tires somewhere between six and ten years old even if tread remains. Check your tire maker's guidance.
Can I replace just two tires instead of four?
Often, yes, as long as the new pair matches the others in size and type. Most tire professionals recommend putting the newer tires on the rear axle for stability. Many all-wheel-drive vehicles need all four tires closely matched in tread depth, so check your owner's manual before buying a pair.
Are all-season tires good enough for winter?
In mild climates with occasional light snow, many drivers manage with all-seasons. Where winters bring regular snow, ice or temperatures below about 45°F (7°C), dedicated winter tires stop and turn noticeably better. Some places, such as Quebec, legally require winter tires for part of the year.