How to Read Tire Sizes: Complete Guide to Tire Size Numbers

TL;DR Tire size markings like P225/65R17 tell you the tire’s type, width, aspect ratio, construction, and rim diameter in a compact code. Learn what each part means, where to find the size on your vehicle, and how load index and speed rating affect safety and performance so you can buy the right replacement tires every time.

## Tire Size Format Explained (P225/65R17)

When you look at the sidewall of any passenger-car tire you’ll see a string of numbers and letters. Although it looks cryptic, each character has a specific meaning. Using the common example P225/65R17, the format breaks down as follows:

Symbol	Meaning	What It Tells You
P	Tire type	P = Passenger-car tire (other codes: LT = Light Truck, ST = Special Trailer, T = Temporary spare)
225	Section width	Width of the tire measured in millimeters from sidewall to sidewall when mounted on the recommended rim and inflated to proper pressure.
/	Separator	Simply divides width from aspect ratio.
65	Aspect ratio	Ratio of the tire’s sidewall height to its width, expressed as a percentage. A 65 aspect ratio means the sidewall height is 65 % of 225 mm (≈146 mm).
R	Construction type	R = Radial (the overwhelming majority of modern tires). Other codes: D = Diagonal (bias-ply), B = Belted bias.
17	Rim diameter	Diameter of the wheel the tire is designed to fit, measured in inches.

Understanding each piece lets you quickly compare tires, spot mismatches, and know what to look for when shopping.

## What Each Number Means – Detailed Explanation with Visuals Described

1. Tire Type (Letter Prefix)

P – Passenger car. Designed for comfort, fuel efficiency, and everyday driving.
LT – Light-truck. Built with stronger sidewalls for heavier loads and off-road use.
ST – Special trailer. Optimized for trailer axles, not for drive wheels.
T – Temporary spare (“donut”). Intended for short-term use only.

If you see no letter at the start (e.g., 225/65R17), the tire is assumed to be a Metric or Euro-metric passenger tire, which is functionally the same as a “P” tire for most consumer applications.

2. Section Width (Three-Digit Number)

The width is measured at the tire’s widest point, perpendicular to the direction of travel. A wider tire generally offers more grip but can increase rolling resistance and fuel consumption.

Visual description: Imagine looking at the tire head-on. The distance from the outer edge of the left sidewall to the outer edge of the right sidewall, measured in millimeters, is the section width.

3. Aspect Ratio (Two-Digit Number After the Slash)

This number expresses the sidewall height as a percentage of the width. Lower aspect ratios (e.g., 45 or 50) mean a shorter sidewall, which yields sharper handling and a sportier feel but a harsher ride. Higher ratios (70-80) give a taller sidewall, more cushioning, and better comfort on rough roads.

Visual description: Picture a cross-section of the tire. Draw a line from the rim edge straight up to the tread; that’s the sidewall height. The aspect ratio tells you how tall that line is compared to the width.

4. Construction Type (Letter After Aspect Ratio)

R (Radial) – Plies run radially from bead to bead, at 90° to the direction of travel. This design provides flexibility, lower heat buildup, and better fuel economy.
D (Diagonal/Bias-ply) – Plies criss-cross at angles (typically 30-45°). Older technology, found mostly on specialty or vintage tires.
B (Belted Bias) – Similar to diagonal but with reinforcing belts under the tread for added strength.

5. Rim Diameter (Two-Digit Number After Construction)

This is the inner diameter of the tire that matches the wheel’s outer diameter, measured in inches. It’s critical: a tire marked 17 will not safely mount on a 16-inch or 18-inch rim.

## Load Index and Speed Rating

Beyond the basic size, tires carry a load index (numeric) and a speed rating (letter) that together define how much weight the tire can safely support and the maximum speed it is rated for. These appear immediately after the size, e.g., 225/65R17 95H.

Load Index Table (selected values)

Load Index	Maximum Load (lbs)	Maximum Load (kg)
71	761	345
75	852	386
80	992	450
85	1,135	515
90	1,323	600
91	1,356	615
95	1,521	690
100	1,764	800
105	2,039	925
110	2,337	1,060

(Source: NHTSA tire safety guidelines)

Speed Rating Table (selected values)

Speed Rating	Maximum Speed (mph)	Maximum Speed (km/h)
L	75	120
M	81	130
N	87	140
P	93	150
Q	99	160
R	106	170
S	112	180
T	118	190
U	124	200
H	130	210
V	149	240
W	168	270
Y	186	300
(Z)	149+	240+

Note: The speed rating is the maximum speed the tire can sustain under optimal conditions; it does not imply you should drive at that speed. Always follow posted limits and vehicle manufacturer recommendations.

How to read them together: In 225/65R17 95H, the tire can carry up to 1,521 lb (690 kg) per tire and is rated for speeds up to 130 mph (210 km/h).

## How to Find Your Tire Size

Knowing where to locate the correct size prevents guesswork and ensures you purchase tires that match your vehicle’s specifications.

Driver-side door jamb placard
- Open the driver’s door and look at the sticker affixed to the door frame (often called the “tire information label”).
- It lists the recommended tire size, inflation pressure, load index, and speed rating for front and rear axles (if they differ).
Owner’s manual
- The maintenance or specifications section includes a tire size chart.
- Useful if the placard is missing or illegible.
Current tire sidewall
- If you’re replacing a tire and the existing ones are still the correct size, read the markings directly.
- Verify that the size matches the placard; mismatches may indicate a previous incorrect installation.
Vehicle identification number (VIN) lookup
- Some online tools let you enter your VIN to retrieve factory tire specs.

Pro tip: Always cross-check at least two sources (placard + manual or sidewall) before buying.

## Tire Size Comparison Chart

Below is a quick reference for common passenger-car sizes, showing the approximate overall diameter and circumference. These values help you understand how a change in width, aspect ratio, or rim size affects rolling distance—important for speedometer accuracy and clearance.

Tire Size	Section Width (mm)	Aspect Ratio	Rim Diameter (in)	Overall Diameter (in)	Circumference (in)
185/65R15	185	65	15	24.5	76.9
195/60R15	195	60	15	24.2	76.0
205/55R16	205	55	16	24.9	78.2
215/55R17	215	55	17	26.3	82.6
225/60R16	225	60	16	27.6	86.7
225/65R17	225	65	17	28.5	89.5
235/45R18	235	45	18	26.3	82.6
245/40R19	245	40	19	26.7	83.9
255/35R20	255	35	20	26.0	81.7
265/30R22	265	30	22	28.0	88.0

How to use the chart: If you consider moving from a 205/55R16 to a 225/60R16, note that the overall diameter grows from 24.9 in to 27.6 in—a ~10.8 % increase. This will cause your speedometer to read slower than actual speed unless you recalibrate.

## FAQ

Q1: What does 225/65R17 mean? A: It denotes a passenger-car tire (P implied) that is 225 mm wide, has a sidewall height equal to 65 % of that width, uses radial construction, and fits a 17-inch rim.

Q2: What does the “R” stand for? A: “R” indicates radial construction, the standard for virtually all modern passenger tires. Radial tires have plies that run perpendicular to the direction of travel, offering better ride comfort, fuel efficiency, and tread life than bias-ply designs.

Q3: How do I know what size tire to buy for my car? A: Check the driver-side door jamb placard or your owner’s manual for the manufacturer-recommended size. Verify the load index and speed rating meet or exceed those specifications. If you want to deviate (e.g., for performance or aesthetics), ensure the new overall diameter stays within ±3 % of the original to avoid speedometer error and clearance issues.

Q4: Can I use a different tire size than what’s on the placard? A: Yes, but with caution. You may go wider or taller as long as:

The tire clears the wheel well, suspension components, and steering linkage at full lock and over bumps.
The load index is equal to or greater than the original.
The speed rating is equal to or greater than the original (unless you never drive near the tire’s limit).
The overall diameter change stays within roughly ±3 % to keep speedometer and odometer accurate. When in doubt, consult a tire professional or use an online tire-size calculator.

Q5: Do I need to replace all four tires at once? A: Ideally, yes—especially on all-wheel-drive (AWD) or four-wheel-drive (4WD) vehicles—to prevent drivetrain strain from differing tire diameters. If you must replace only two, mount the new tires on the rear axle (for better stability) and ensure they match the existing tires in size, load index, speed rating, and tread pattern as closely as possible.

Q6: How often should I check tire pressure? A: At least once a month and before long trips. Use the pressure listed on the placard (not the maximum pressure molded into the sidewall). Proper inflation maximizes tire life, fuel efficiency, and safety.

Callout

Compare tire sizes with our free Tire Size Calculator

Use the tool to input your current tire size and see how alternative widths, aspect ratios, or rim diameters affect overall diameter, circumference, and speedometer error.

Additional Resources

NHTSA Tire Safety: https://www.nhtsa.gov/equipment/tires – Official guidelines on tire maintenance, load limits, and speed ratings.
Unit Converter – Handy for switching between metric and imperial measurements when comparing tire specs.

Keeping your tires in the right size and condition is one of the simplest ways to improve safety, fuel economy, and driving enjoyment. With the knowledge above, you can confidently decode any sidewall marking and choose the perfect replacement for your vehicle.