How To Read A Tire Size - Popular Mechanics

Maybe your like

Newsletter
Pop Mech Pro
Subscribe
Home and DIY
Power Tools
Cars
Technology
Military
Science
Adventure
Sci-Fi
Airplanes
Space
Gifts
Deals
Visual Stories
Promotions
Shop
Follow
My Bookmarks

Skip to Contentsign inSubscribeSearch

sign inSubscribe

Science
Home & DIY
Technology
Gear Reviews
Military
Transportation
Culture & Curiosities
About

My Bookmarks
Newsletter

Privacy NoticeTerms Of Use

sign inSubscribe

Science
Home & DIY
Technology
Gear Reviews
Military
Transportation
Culture & Curiosities
About
More

Upgrading your vehicle's wheels and tires is a popular modification that can improve the look and performance of your ride, and picking the right tire can be easy—if you know where to look.

The first step is to determine what size wheels and tires you currently have and then decide how big or small you want to go. A size is always shown on the sidewall of any tire. Passenger vehicle tires are usually in metric form and larger truck tires can be shown as metric or standard size.

This diagram explains what each number or letter means.

1. Prefix

P = Passenger car

T = Temporary spare

LT = Light truck

ST = Trailer

2. Section width

That`s the overall width of the tire in millimeters when mounted on the correct width rim.

3. Aspect ratio

This indicates the distance from the wheel-rim mounting surface to the outside of the tread. Aspect ratio is expressed as a proportion of the width instead of as an absolute number, so it's easier to compare the profiles of different-size tires. A 50-series tire is half as tall as wide, while a 70-series is 70 percent as tall as its width.

4. Speed rating

Only the highest-rated tires, the Z-rated models, still label the speed rating inside the tire-size designation. On all other tires, it's found elsewhere on the sidewall.

5. Construction

R = Radial-ply tire.

D = Refers to diagonal, or old-fashioned bias-ply tire, now used only on trucks and trailers.

6. Size

The diameter of the rim in inches. If it seems odd to you to spec tire width in millimeters and diameter in inches, it does to me, too. A few special-purpose tires, like PAX system tires, use a metric designation for rim diameter.

A standard sized tire will be displayed in inches. A tire that reads 33 x 12.5 R15, is 33-inches in diameter (or tall), 12.5-inches wide, and will fit a 15-inch wheel.

The door jam of your vehicle will also indicate what size tires the manufacturer recommends. Going beyond that size will throw off your gearing and result in slower acceleration. It will also change your speedometer reading. Most tire shops will have a maximum wheel and tire size listed for your vehicle that will still fit without rubbing, but it's always best to check multiple sources and take advantage of online forums for advice.

Check out our guide for upgrading wheels and tires because bigger isn't always better.

Timothy DahlDIY Editor

Timothy is a lifelong DIY enthusiast who is fixated on smart home tech, beautiful tools, and wrenching on his FJ62 Land Cruiser. He’s the DIY editor at Popular Mechanics and also the founder of the home improvement site, Charles & Hudson, the Webby-nominated family site, Built by Kids, and Tool Crave. When not working, you’ll find him on his board or bike enjoying the LA weather with his family and friends. Follow him on Instagram and Twitter.

Watch Next Advertisement - Continue Reading Below