How to Read Tire and Wheel Size: A Complete Guide to the Numbers on Your Tires
How to Read Tire and Wheel Size: A Complete Guide to the Numbers on Your Tires
Every tire has a string of numbers and letters stamped on its sidewall — something like 225/45R17 91H. If that looks like a foreign language, you're not alone. But once you understand what each part means, you'll be able to buy the right tires and wheels with confidence. This guide walks you through every character, step by step.
Breaking Down the Tire Size Code
Let's use 225/45R17 91H as our example and decode it piece by piece.
Section Width (225)
The first number is the tire width in millimeters, measured from sidewall to sidewall when the tire is mounted on its recommended rim width. A 225mm tire is roughly 8.9 inches wide. Wider tires generally provide more grip, while narrower tires can improve fuel efficiency.
Aspect Ratio (45)
The number after the slash is the aspect ratio — the height of the sidewall expressed as a percentage of the width. A 45 aspect ratio means the sidewall height is 45% of 225mm, which equals about 101mm (roughly 4 inches).
Lower aspect ratios (like 35 or 40) give you a sportier look and sharper handling response. Higher ratios (55, 60, 65) provide a more comfortable ride and better pothole absorption — which matters a lot when you're protecting expensive OEM wheels.
Construction Type (R)
The letter indicates how the tire is built internally:
- R = Radial — by far the most common. Internal plies run perpendicular to the direction of travel for a smoother ride and longer tread life.
- D = Diagonal (bias-ply) — found on some trailers and vintage vehicles.
- B = Bias belt — rare; mostly specialty and aircraft tires.
You might also see RF (run-flat) or ZR (speed-rated above 149 mph) in this position.
Rim Diameter (17)
This is the big one for wheel shoppers: the diameter of the wheel in inches that the tire is designed to fit. If your tire says R17, you need a 17-inch wheel. This number is essential when ordering replacement OEM wheels — get it wrong and the tire simply won't mount.
Common OEM wheel diameters range from 15 inches on compact cars up to 22 inches on full-size SUVs and trucks. Some performance and luxury vehicles now come with 23- or even 24-inch factory wheels.
Load Index (91)
The load index tells you how much weight each tire can support when properly inflated. It's not a direct pounds figure — it's a code that corresponds to a specific weight capacity. A load index of 91 means each tire supports up to 1,356 lbs.
Here are some common load index values:
| Load Index | Max Load (lbs) | Load Index | Max Load (lbs) |
|---|---|---|---|
| 75 | 853 | 100 | 1,764 |
| 80 | 992 | 105 | 2,039 |
| 85 | 1,135 | 110 | 2,337 |
| 87 | 1,201 | 115 | 2,679 |
| 90 | 1,323 | 120 | 3,086 |
| 91 | 1,356 | 125 | 3,638 |
| 95 | 1,521 | 130 | 4,189 |
Speed Rating (H)
The final letter indicates the maximum sustained speed the tire is engineered to handle safely. Higher-rated tires generally offer better grip and heat dissipation at speed.
| Rating | Max Speed | Typical Vehicles |
|---|---|---|
| S | 112 mph | Family sedans, minivans |
| T | 118 mph | Sedans, SUVs |
| H | 130 mph | Sport sedans, coupes |
| V | 149 mph | Sports cars, performance sedans |
| W | 168 mph | High-performance sports cars |
| Y | 186 mph | Supercars, exotic vehicles |
Pro tip: If your vehicle has different speed ratings on the front and rear tires (common on staggered setups), always drive according to the lower-rated tire for safety.
Other Numbers You'll Find on Your Wheel
Beyond tire markings, the wheel itself carries critical specs:
- Bolt Pattern — The number of lug holes and the diameter of the circle they form (e.g., 5×114.3mm). This must match your vehicle exactly.
- Offset (ET) — Measured in millimeters, this determines how far the wheel sits inward or outward from the hub face. Wrong offset causes rubbing, uneven tire wear, and handling problems.
- Hub Bore — The center hole diameter. OEM wheels are hub-centric (exact fit), while aftermarket wheels often use hub rings to adapt.
Frequently Asked Questions
What does the "R" mean in tire size?
The "R" stands for radial construction, meaning the tire's internal cord plies are arranged in a radial pattern (perpendicular to the tread). Radial tires make up over 98% of all passenger vehicle tires sold today because they offer better grip, longer life, and improved fuel economy compared to bias-ply designs.
How do I find my wheel size without looking at the tire?
Check your vehicle's owner's manual, the sticker on the driver's side door jamb, or look up your year/make/model on the manufacturer's website. You can also search your vehicle on OEM Rim Shop to find the exact factory wheel specs.
Can I put a different tire size on my OEM wheels?
You can adjust the tire width and aspect ratio within a narrow range — known as "plus sizing" or "minus sizing" — as long as the overall tire diameter stays within 3% of the original. Going outside that range affects your speedometer accuracy, ABS calibration, and ride quality.
What happens if I use the wrong load index?
Running tires with a load index lower than your vehicle's specification is dangerous. The tires may overheat under normal driving conditions, leading to blowouts. Always match or exceed the load index listed in your owner's manual.
Why This Matters When Buying Wheels
Understanding tire and wheel sizing isn't just trivia — it directly affects your safety, ride quality, and wallet. When you shop for replacement OEM wheels, knowing your rim diameter, bolt pattern, and offset means you'll get a wheel that fits perfectly without any guesswork.
Need help finding the right OEM wheel for your vehicle? Browse our inventory at OEM Rim Shop — every wheel is listed with full specifications so you can match it to your tire size with confidence.
