How to Read Tire Size: What 225/65R17 Actually Means
How to read tire size, one piece at a time: width, aspect ratio, R, rim size, load index and speed symbol, plus LT and flotation sizes like 31x10.50R15.
By TireDrop Team, Extreme Tires, Sunrise FLPublished Updated 4 min read
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.
| Piece | In the example | What it means | What it changes on the road |
|---|---|---|---|
| Width | 225 | Section width in millimeters, at the widest point | Contact patch width and whether it fits the wheel well |
| Aspect ratio | 65 | Sidewall height as a percent of the width | Ride comfort, sidewall flex, overall diameter |
| Construction | R | Radial, the standard build on modern passenger tires | Nothing to choose: nearly everything is R |
| Rim diameter | 17 | Wheel diameter in inches; must match the wheel exactly | Which wheels the tire can mount on |
| Load index | 102 | A code for the maximum load one tire can carry when properly inflated | Whether the tire can carry your vehicle |
| Speed symbol | H | The maximum sustained speed the tire is rated for; H is 130 mph | A test rating, not a suggested driving speed |
Type your own size into the decoder. It highlights each piece and explains it in one line.
Try it
Tire size decoder
Type the size from your sidewall or door placard to see what each part means.
Metric (225/45R17 94W), light truck (LT265/70R17 121/118S) and flotation (33x12.50R20) sizes all work.
6 parts decoded. Sidewall 101 mm (4.0 in), overall diameter 25.0 in (634 mm).
Tap a part to highlight its meaning
- Section width225
- 225 is the section width in millimeters (about 8.9 in), measured sidewall to sidewall on a mounted, inflated tire.
- Aspect ratio45
- 45 is the aspect ratio: the sidewall is 45% as tall as the tire is wide, about 101 mm (4 in).
- ConstructionR
- R means radial construction: the body cords run straight across the tire from bead to bead, under steel belts. Nearly every road tire sold today is a radial.
- Wheel diameter17
- 17 is the wheel diameter in inches. The tire mounts only on a 17-inch wheel.
- Load index94
- 94 is the load index: a maximum of 1,477 lb (670 kg) per tire. Replacement tires should match or exceed the load index your vehicle maker specifies (door placard or owner's manual).
- Speed ratingW
- W is the speed rating: the tire passed a lab test at up to 168 mph. It's a test rating, not a recommended driving speed.
Sidewall height
101 mm (4.0 in)
Overall diameter
25.0 in (634 mm)
Nominal figures worked out from the size. Real dimensions vary a little by tire model and wheel width. When buying, start from the size on your door placard.
Sources: NHTSA, Tire Safety: Everything Rides On It (opens in a new tab); Michelin, Tire Markings Explained (opens in a new tab); Michelin, Load Rating & Speed Rating (opens in a new tab); Goodyear, Tire Load Index (opens in a new tab)
Text version of this demo
A size such as 225/45R17 94W breaks down into section width in millimeters (225), aspect ratio (45), radial construction (R), wheel diameter in inches (17), load index (94) and speed rating (W). The demo also works out sidewall height and overall diameter, including LT and flotation sizes like 33x12.50R20.
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.
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. 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:
| Symbol | Rated speed |
|---|---|
| H | 130 mph |
| V | 149 mph |
| W | 168 mph |
| Y | 186 mph |
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. According to Discount Tire, a flotation size reads in inches from 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 31x10.50R15LT, marking them as light truck tires.
Three mistakes we'd rather you skip
- Buying by the first number. 225 tells you width only. Aspect ratio and rim size decide whether the tire will actually fit.
- Copying the old tires. If the previous owner put on a different size, you'll repeat the mistake. Check the placard.
- Ignoring the letters. A lower load index or speed symbol than the placard calls for isn't a like-for-like replacement.
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.
Questions people ask
What does 225/65R17 mean on a tire?
225 is the section width in millimeters. 65 is the aspect ratio, meaning the sidewall is 65% as tall as the tire is wide. R means radial construction. 17 is the wheel diameter in inches that the tire fits.
What does the R mean on a tire?
R stands for radial construction, the internal build used on modern passenger tires. It is not the rim size and it is not a speed rating.
Where do I find my tire size?
Your vehicle's recommended tire size is on the Tire and Loading Information placard, usually on the driver's door edge or door post, and in the owner's manual. The size molded on your current tires tells you what is on the car now, which may not match.
What does LT mean on a tire size?
LT marks a light truck tire. Other letters you may see are P for passenger car, T for a temporary spare and ST for a special trailer tire. No letter usually means a metric size built to European ETRTO standards.
Sources
- 1.How to Read Tire Markings and Sidewall Codes (opens in a new tab) — Michelin
- 2.Tire Load Rating & Speed Rating Explained (opens in a new tab) — Michelin
- 3.Tire Safety: Everything Rides On It (opens in a new tab) — NHTSA
- 4.Replacing Tires (opens in a new tab) — U.S. Tire Manufacturers Association
- 5.Flotation Tires vs Metric Tires (opens in a new tab) — Discount Tire
Related reading
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