How to Read Tire Size: What 225/65R17 Actually Means

Start at the door, not the tire

Before you decode anything, find the right size to decode. There are two places a tire size lives, and they don't always agree.

• On the tire. This tells you what's on the car right now. If you bought the car used in Hialeah or Hollywood, someone may have changed it.
• On the placard. NHTSA describes the Tire and Loading Information label, usually on the driver's side door edge or door post, and the owner's manual. This tells you what the vehicle maker specified.

USTMA's standard practice is that replacement tires match the placard: the same size, load-carrying capacity and speed rating the vehicle maker lists. So when the tire and the placard disagree, the placard wins until someone qualified tells you otherwise.

225/65R17 102H, piece by piece

Here's a typical SUV size, broken down using Michelin's definitions.

Type your own size into the decoder. It highlights each piece and explains it in one line.

The letter in front: P, LT, T, ST or nothing

Some sizes start with a letter. NHTSA's tire safety material lists what they mean:

• P: passenger car
• LT: light truck
• T: temporary (your compact spare)
• ST: special trailer

And if there's no letter at all? Michelin explains that a leading P means the tire's loads follow U.S. Tire and Rim Association standards, while no letter means the tire conforms to the European ETRTO standards. You'll see both on cars in the same Doral parking lot.

Don't swap between P, LT and ST types on your own. They're different service types for different vehicles, so treat a change of type as a fitment question for a technician. Our guide to LT vs P-metric tires explains what changes.

Doing the math: how tall is the sidewall?

The aspect ratio is a percentage, which trips people up. Here's how to turn it into inches.

Sidewall height = width × aspect ratio 225 mm × 0.65 = 146.25 mm, or about 5.76 in

Overall diameter = rim + two sidewalls 17 in + (2 × 5.76 in) = about 28.5 in

Now compare a size that looks similar on paper, 225/45R19:

• Sidewall: 225 × 0.45 = 101.25 mm, about 3.99 in
• Overall diameter: 19 + (2 × 3.99) = about 27.0 in

Same width, but the second tire is about an inch and a half shorter overall with a much thinner sidewall. That's why sizes aren't interchangeable just because the first number matches. Overall diameter affects speedometer readings and clearance. Our guide on using a different tire size covers that trade-off.

The tire size tool does this math for you and can compare two sizes side by side.

Load index and speed symbol

The two characters after the size are easy to skip and important not to.

Load index. A number that maps to how much weight one tire can carry at proper inflation. Michelin's chart lists load index 94, for example, as 670 kg, or about 1,477 lb, per tire. USTMA says to always use a load index equal to or above what your vehicle placard requires. Never go lower. You'll sometimes see XL after it, which marks an extra-load version of the size; our XL vs SL guide explains how it ties to pressure. The load index guide has the full chart.

Speed symbol. A letter for the maximum sustained speed the tire is rated to carry its load at. From Michelin's chart:

A speed rating is a lab rating under set conditions, not a target for the Sawgrass Expressway. It matters when you buy: USTMA's guidance is to match the speed rating the vehicle maker recommends.

Flotation sizes: 31x10.50R15 and friends

Trucks and off-road builds sometimes use flotation sizes, which look nothing like metric ones. Bridgestone describes high flotation sizing for light trucks as overall diameter in inches, then section width in inches, then construction type, then rim diameter in inches. Read left to right:

• 31: overall diameter, 31 inches tall
• 10.50: section width, 10.5 inches
• R: radial
• 15: rim diameter, 15 inches

So with a flotation size, you get the overall diameter up front without doing any math. You'll often see these with LT after the numbers, as in Bridgestone's example 33x12.50R15LT, marking them as light truck tires.

Three mistakes we'd rather you skip

1. Buying by the first number. 225 tells you width only. Aspect ratio and rim size decide whether the tire will actually fit.
2. Copying the old tires. If the previous owner put on a different size, you'll repeat the mistake. Check the placard.
3. Ignoring the letters. A lower load index or speed symbol than the placard calls for isn't a like-for-like replacement.

Driving a sedan? Our Toyota Camry tire guide walks through size, noise and fuel economy for a commuter car.

Know your size? Find tires by vehicle or shop by size. Not sure what's on the placard versus the car? Call Extreme Tires at (954) 773-1896 and ask.