Every tire sold in the United States carries a code stamped directly onto its sidewall — something like P215/65R15 95H. At first glance, it looks like a random mix of letters and numbers. In reality, it’s a compact spec sheet that tells you the tire’s width, height, construction type, wheel diameter, load capacity, and maximum safe speed.

Knowing how to read tire size numbers matters for more than curiosity. It protects you from buying the wrong tire, helps you avoid handling and safety issues, and can even save you money by preventing a mismatched purchase. This guide walks through every part of the code step by step, with real examples, a labeled diagram description, and answers to the most common questions people search for.

Where to Find Your Tire Size

Before decoding the numbers, you need to know where to look. There are two reliable places:

• The tire sidewall itself — molded directly into the rubber on the outer edge of the tire.
• The driver’s door jamb sticker — open the driver’s door and look at the frame; the automaker lists the recommended factory tire size and cold inflation pressure here.

You can also check the vehicle’s owner’s manual. Note that the size on your current tires may not always match the factory-recommended size if a previous owner upgraded the wheels — always cross-check with the door jamb sticker if you’re unsure which size is correct for your car.

Tire Size Number Breakdown (Section by Section)

Let’s use a real example throughout this section: P215/65R15 95H.

1. Tire Type Letter (P, LT, ST, T)

The first letter (or letters) identifies what class of vehicle the tire is designed for:

• P — Passenger vehicle (cars, minivans, small SUVs)
• LT — Light Truck (heavier-duty, used for towing or hauling)
• ST — Special Trailer (designed for trailers, not for steering axles)
• T — Temporary (spare tire only)
• No letter at all — this is a “Euro-metric” or “flotation” size, common on many modern SUVs and light trucks

2. Width in Millimeters (215)

This number is the tire’s width in millimeters, measured from one sidewall to the other at the widest point. A larger number means a wider tire, which generally improves grip and cornering but can affect fuel economy and ride comfort.

3. Aspect Ratio (65)

This is the sidewall height, expressed as a percentage of the tire’s width. In this example, 65 means the sidewall height equals 65% of the 215mm width (about 140mm). Lower aspect ratios (like 40 or 45) mean a shorter, stiffer sidewall typically found on sportier vehicles; higher aspect ratios (like 70 or 75) give a taller, more cushioned sidewall common on trucks and family sedans.

4. Construction Type (R)

The “R” stands for Radial, meaning the internal fabric plies run radially (perpendicular) across the tire. Nearly all tires sold today are radial construction. You may occasionally see “D” (diagonal/bias-ply) on trailer or specialty tires, or “B” (belted bias) on older vehicles.

5. Wheel/Rim Diameter (15)

This number tells you the diameter, in inches, of the wheel the tire is designed to fit — in this case, a 15-inch rim. This must match your wheel exactly; there is no flexibility here.

6. Load Index (95)

A numeric code that corresponds to the maximum weight the tire can safely carry when properly inflated. Covered in detail in the next section.

7. Speed Rating (H)

A letter code indicating the maximum sustained speed the tire is rated for. Also covered in detail below.

Load Index & Speed Rating Explained

These two codes are often overlooked, but they directly affect safety and are legally important to match to your vehicle’s requirements.

Common Load Index Values

Common Speed Rating Letters

Note: These ratings represent the tire’s engineered capability under controlled test conditions — not a suggestion to drive at these speeds. Always follow posted speed limits.

Passenger, LT, and Trailer Tire Codes Compared

Not every vehicle uses the same numbering pattern. Here’s how the three most common formats differ:

If you’re shopping for a trailer tire specifically, our 205/75R15 Trailer Tire Guide walks through everything you need to know about this exact size.

Common Mistakes When Reading Tire Sizes

• Confusing width with diameter. The “215” is the width in millimeters, not the wheel size — the wheel diameter is the number right after the “R” (e.g., the “15”).
• Ignoring the load index. A tire with a lower load index than your vehicle requires can be dangerously overloaded, especially with passengers and cargo.
• Mixing tire types on one axle. Combining P-metric and LT tires, or radial and bias-ply tires, on the same axle can create handling instability.
• Assuming bigger is always better. A wider tire or larger rim diameter can rub against the wheel well or throw off the speedometer if it doesn’t match the vehicle’s designed overall diameter.
• Overlooking the sidewall’s actual physical condition. Even if the size is correct, cracked or aging rubber should be inspected — a tire tread depth gauge can help you check remaining life before you rely on old tires.

Practical Tips Before Buying New Tires

1. Always match load index and speed rating to the factory sticker, never go below what’s specified.
2. Check tire pressure regularly with a reliable tire pressure gauge — correct inflation affects both safety and the accuracy of the load rating.
3. Inspect valve stems when replacing tires; a worn valve stem is a common source of slow leaks. See our tire valve stem guide for details.
4. Keep a basic repair kit on hand. A tire plug kit or tire patch kit can get you out of a roadside puncture safely.
5. Have the right tools. A quality tire iron and, for shops or serious DIYers, a manual tire changer make swaps far easier.
6. Protect your valve stems from the elements with proper tire caps.
7. Buy from a trusted local source. Browse tire brands or find a certified installer through our services directory, searchable by state.

Frequently Asked Questions

What do the numbers on the side of a tire mean?

They represent the tire’s type, width in millimeters, aspect ratio (sidewall height as a percentage of width), construction type, rim diameter in inches, load index, and speed rating — in that order.

How do I know what size tire fits my car?

Check the sticker inside the driver’s door jamb or the owner’s manual for the manufacturer’s recommended size, then match any replacement tire to that exact width, aspect ratio, and rim diameter.

Can I put a wider tire on my car?

Sometimes, within limits set by the wheel width and vehicle clearance, but it’s best to consult a tire professional first since it can affect handling, speedometer accuracy, and warranty coverage.

What does the last letter in a tire size mean?

The final letter is the speed rating, indicating the maximum speed the tire can safely sustain, such as “H” for up to 130 mph.

Do tire size numbers use inches or millimeters?

It’s a mix: the width is in millimeters, the aspect ratio is a percentage, and the rim diameter is in inches.

Conclusion

Reading tire size numbers doesn’t require any special training once you know the pattern: Type, Width, Aspect Ratio, Construction, Rim Diameter, Load Index, and Speed Rating. Whether you’re replacing a worn set, upgrading your wheels, or simply double-checking that your spare matches your vehicle, this code on the sidewall gives you everything you need to make a safe, informed choice. Always cross-reference with your door jamb sticker before buying, and when in doubt, ask a certified tire professional to confirm fitment.

For more hands-on guides — from choosing a tire plug kit to finding a shop near you — explore the rest of the TireNearMeHub blog.