Car Tire Guide: How to Choose, Maintain, and Know When to Replace Them

Tires are the only part of a car that touches the road, yet they are often checked only when something already feels wrong. Manufacturers and industry groups publish specific numbers for pressure, tread depth, rotation and tire age — this guide collects that guidance in one place.

How to read a tire size and choose the right replacement

Every tire sidewall carries a size code such as 225/45R18 95H. According to Continental's tire-knowledge guide, the first number (225) is the tire's width in millimetres, the two digits after the slash (45) are the aspect ratio — sidewall height as a percentage of width — and R marks radial construction. The following number (18) is the wheel diameter in inches, while the last number-letter pair (95H) is the load index and speed rating.

Tire maker Yokohama states that replacement tires should match the original equipment in size, tread design, construction, load index and speed rating, and that the load index and speed rating of a replacement tire must equal or exceed those of the original. The U.S. Tire Manufacturers Association (USTMA) adds that drivers should follow the inflation pressure printed on the vehicle's tire placard, door jamb or owner's manual rather than the number molded onto the tire itself, which is only the tire's maximum pressure.

Summer, winter and all-season tires

Michelin's buying guide identifies 45°F (about 7°C) as the practical switching point between tire types: below that temperature, summer and all-season rubber compounds begin to stiffen and lose grip, while winter-tire compounds are formulated to stay flexible. Michelin recommends switching to winter tires in late fall once average daily temperatures drop below 45°F, and back to summer or all-season tires once spring temperatures consistently rise above it.

Michelin also distinguishes the Three-Peak Mountain Snowflake (3PMSF) symbol, which certifies that a tire passed a standardized severe-snow traction test, from the more basic M+S (Mud and Snow) marking, which does not guarantee the same cold-weather performance.

Checking pressure and tread depth

USTMA recommends checking tire pressure at least once a month, and before long trips, towing or carrying heavy loads, and doing so when tires are cold — meaning the vehicle has not been driven for at least three hours. Inflation pressure can change by one to two pounds per square inch for every 10°F change in outside temperature, so pressure should be rechecked when the weather shifts.

For tread, USTMA and Yokohama both set the legal replacement threshold at 2/32 of an inch (1.6 mm). Tires have built-in wear-bar indicators that become flush with the tread surface at that depth; the classic penny test — inserting a penny upside down into a tread groove — shows the same limit, since Lincoln's head becomes fully visible once tread reaches 2/32 inch. Industry sources note that wet and snow traction can decline measurably before tread reaches the legal minimum, so some drivers choose to replace tires earlier for wet or winter driving.

Rotation and alignment

Michelin explains that front tires typically wear faster than rear tires because of steering, braking and cornering forces, which is why periodic rotation is recommended. Michelin's own interval is every 5,000 to 7,000 miles or every other oil change, while USTMA's general recommendation — to be used only when the vehicle manufacturer gives no specific interval — is every 5,000 to 8,000 miles.

Rotation patterns differ by drivetrain, according to Michelin: front-wheel-drive vehicles move the front tires straight back and cross the rear tires to the front; rear-wheel-drive vehicles do the opposite; all-wheel and four-wheel-drive vehicles typically use a crisscross pattern; and directional tires, designed to spin in one direction only, are rotated front-to-back on the same side of the car. USTMA adds that alignment should be checked if the vehicle pulls to one side, since hitting potholes or curbs can knock wheels out of alignment and cause uneven wear.

When damage — not just wear — means replacement

Yokohama's guidance states that if a tire is suspected of being damaged by a road hazard such as a pothole, glass, metal or a curb, drivers should stop using it immediately, switch to a properly inflated spare, and have the tire inspected inside and out by a qualified technician before returning it to service. The same guidance recommends replacing tires in full sets of matching size, tread design and rating, and installing new tires on the rear axle if only two are being replaced at once.

1. Find your vehicle's required tire size, load index and speed rating on the door-jamb placard or in the owner's manual.
2. Check inflation pressure monthly when tires are cold, matching the vehicle placard figure, not the number on the tire sidewall.
3. Check tread depth monthly using the wear-bar indicators or a penny; replace at 2/32 inch (1.6 mm) or sooner for wet/winter driving.
4. Rotate tires on the schedule set by the vehicle manufacturer, or every 5,000–8,000 miles if none is specified.
5. Have alignment checked if the car pulls to one side or tread wears unevenly.
6. Switch to winter tires once average temperatures drop below about 45°F (7°C), and back to summer/all-season tires in spring.
7. Replace a tire immediately, and have it inspected, after any suspected impact with a pothole, curb or road debris.
8. When buying replacements, match size, load index and speed rating at minimum, and replace in full matching sets where possible.