How to Read Tire Size: A Complete Guide to Tire Markings

Q: What does the R mean in tire size?

The 'R' in a tire size designation like P225/65R17 stands for Radial construction. In a radial tire, the internal cord plies run perpendicular (at 90 degrees) to the direction of travel, from bead to bead across the tread. Radial construction provides better fuel economy, longer tread life, improved ride comfort, and superior handling compared to older bias-ply designs. According to the Rubber Manufacturers Association, over 98% of all passenger car tires sold today are radial construction. You may occasionally see 'B' for bias-belt construction or 'D' for diagonal (bias-ply) construction on specialty or vintage tires, but these are uncommon for modern passenger vehicles.

Updated March 2026 · 10 min read

Compare tire sizes instantly

Enter two tire sizes to compare overall diameter, width, sidewall height, and speedometer difference.

Open Tire Size Calculator →

How Tire Size Markings Work

Tire size markings are a standardized code printed on every tire sidewall that describes the tire's width, height, construction type, rim diameter, load capacity, and maximum speed rating. Reading these markings correctly is essential for buying replacement tires, comparing tire options, and ensuring your vehicle operates safely and efficiently.

The tire sizing system used in North America follows standards set by the U.S. Tire Manufacturers Association (USTMA), formerly the Rubber Manufacturers Association. According to the USTMA, approximately 300 million replacement tires are sold in the United States each year, making tire size one of the most commonly referenced vehicle specifications. A typical tire size looks like this: P225/65R17 95H.

Each segment of this code conveys specific information. Understanding what each number and letter means helps you choose the right tires, compare alternatives, and verify compatibility with your vehicle's wheel size. The National Highway Traffic Safety Administration (NHTSA) recommends always replacing tires with the size specified by the vehicle manufacturer, which is printed on a sticker inside the driver's door jamb.

Decoding Each Part of a Tire Size

Let us break down the tire size P225/65R17 95H piece by piece:

P225/65R17 95H

P = Tire Type — "P" stands for Passenger vehicle. "LT" would indicate Light Truck.
225 = Section Width — The tire width in millimeters from sidewall to sidewall when mounted.
65 = Aspect Ratio — The sidewall height as a percentage of the section width. 65% of 225 mm = 146.25 mm.
R = Construction Type — "R" means Radial. Over 98% of modern tires are radial.
17 = Rim Diameter — The wheel diameter in inches that this tire fits.
95 = Load Index — The maximum weight each tire can support (95 = 1,521 lbs / 690 kg).
H = Speed Rating — The maximum sustained speed (H = 130 mph / 210 km/h).

Key Terms You Should Know

Section Width — The tire's width in millimeters measured from the widest point of the outer sidewall to the widest point of the inner sidewall when mounted on the recommended rim width. Common widths range from 155 mm (subcompact cars) to 315 mm (performance vehicles).
Aspect Ratio — The ratio of the tire's sidewall height to its section width, expressed as a percentage. A lower number means a shorter sidewall. A 45-series tire has a sidewall that is 45% of the width, producing a "low-profile" look with sharper handling but a firmer ride.
Overall Diameter — The total height of the tire from the ground to the top of the tread. This determines speedometer accuracy, ground clearance, and fender clearance. Use the tire size calculator to compute this from any tire size code.
Load Index — A numerical code corresponding to the maximum weight a tire can carry at the inflation pressure specified on the sidewall. The load index must meet or exceed the value specified by your vehicle manufacturer.
Speed Rating — A letter indicating the maximum speed at which the tire can carry its rated load. It does not indicate the top speed of your vehicle but rather what the tire is engineered to handle safely.
Plus Sizing — The practice of fitting larger-diameter wheels with lower-profile tires to maintain approximately the same overall diameter. For example, moving from 225/65R17 to 245/45R19 keeps the diameter similar while changing the wheel and tire proportions.

Speed Rating Reference Table

Speed ratings indicate the maximum sustained speed a tire is designed to handle. These ratings are assigned after controlled laboratory testing under the European Tyre and Rim Technical Organisation (ETRTO) standards. Always choose a speed rating that matches or exceeds your vehicle manufacturer's recommendation.

Speed Rating	Max Speed (mph)	Max Speed (km/h)	Typical Use
Q	99	160	Winter / studded tires
S	112	180	Family sedans, minivans
T	118	190	Family sedans, minivans
H	130	210	Sport sedans, crossovers
V	149	240	Sports cars, performance sedans
W	168	270	High-performance sports cars
Y	186	300	Supercars, exotic vehicles

Load Index Reference Table

The load index number on a tire corresponds to a specific maximum weight capacity per tire. Your replacement tires must have a load index equal to or greater than the original equipment specification.

Load Index	Max Load (lbs)	Max Load (kg)
85	1,135	515
90	1,323	600
95	1,521	690
100	1,764	800
105	2,039	925
110	2,337	1,060
115	2,679	1,215
120	3,086	1,400

How to Calculate Overall Tire Diameter

The overall diameter of a tire determines your effective gear ratio, speedometer accuracy, and ground clearance. Here is the step-by-step calculation for P225/65R17:

Step 1: Sidewall height = Section width × Aspect ratio = 225 mm × 0.65 = 146.25 mm

Step 2: Convert to inches = 146.25 / 25.4 = 5.76 inches

Step 3: Overall diameter = (2 × Sidewall height) + Rim diameter = (2 × 5.76) + 17 = 28.52 inches

Step 4: Circumference = Diameter × π = 28.52 × 3.14159 = 89.6 inches

Step 5: Revolutions per mile = 63,360 / 89.6 = ~707 rev/mile

Practical Examples: Comparing Tire Sizes

Example 1: Upsizing from 16" to 17" Wheels

Upgrading from 215/60R16 to 225/55R17 on a mid-size sedan:

215/60R16: Sidewall = 215 × 0.60 = 129 mm = 5.08" | Diameter = (2 × 5.08) + 16 = 26.16"

225/55R17: Sidewall = 225 × 0.55 = 123.75 mm = 4.87" | Diameter = (2 × 4.87) + 17 = 26.74"

Diameter difference: 0.58" (2.2%) — within the acceptable 3% range.

Speedometer reads: ~1.1% slower than actual speed (reads 60 mph when actually going 60.7 mph).

Example 2: All-Terrain Upgrade for a Truck

Moving from stock 265/70R17 to larger 285/70R17 on a pickup truck:

265/70R17: Sidewall = 265 × 0.70 = 185.5 mm = 7.30" | Diameter = (2 × 7.30) + 17 = 31.60"

285/70R17: Sidewall = 285 × 0.70 = 199.5 mm = 7.85" | Diameter = (2 × 7.85) + 17 = 32.70"

Diameter difference: 1.10" (3.5%) — slightly exceeds the 3% guideline.

Speedometer impact: reads ~1.7% fast (reads 60 mph when actually going 61.0 mph). Consider recalibration.

Example 3: Low-Profile Performance Swap

Going from 225/45R18 to 245/40R18 on a sports sedan:

225/45R18: Sidewall = 225 × 0.45 = 101.25 mm = 3.99" | Diameter = (2 × 3.99) + 18 = 25.98"

245/40R18: Sidewall = 245 × 0.40 = 98 mm = 3.86" | Diameter = (2 × 3.86) + 18 = 25.72"

Diameter difference: 0.26" (1.0%) — excellent match. Width increased by 20 mm for wider contact patch.

Tips for Choosing the Right Tire Size

Always check your door jamb sticker first. The vehicle manufacturer's recommended tire size, load index, and inflation pressure are printed on a label inside the driver's door frame. This is the definitive specification for your vehicle.
Keep overall diameter within 3%. Changing the overall diameter beyond 3% affects speedometer accuracy, ABS calibration, and can void warranty coverage. Use the tire size calculator to compare before purchasing.
Never downgrade load index. Replacement tires must have a load index equal to or greater than the original equipment. Under-rated tires can fail catastrophically under heavy loads, especially on trucks and SUVs.
Match speed ratings across all four tires. Mixing speed ratings can cause unpredictable handling. If you must mix, put the higher-rated tires on the rear axle.
Consider aspect ratio tradeoffs. Lower-profile tires (40–50 series) offer sharper handling and a sportier appearance but ride harsher and are more susceptible to pothole damage. Higher-profile tires (60–70 series) provide a smoother ride and better protection against road hazards.
Check for correct tire pressure. Even the right tire size will perform poorly if inflated incorrectly. According to the NHTSA, about 25% of cars on U.S. roads have at least one significantly underinflated tire.

Other Markings on Your Tire Sidewall

Beyond the main size code, tire sidewalls contain additional important information:

Marking	Meaning	Example
DOT Code	Manufacturing date (last 4 digits = week and year)	DOT xxxx 2523 = week 25 of 2023
Treadwear	UTQG tread life rating (higher = longer lasting)	Treadwear 500 (average is 300–500)
Traction	Wet braking grip rating (AA, A, B, C)	Traction AA (highest)
Temperature	Heat resistance rating (A, B, C)	Temperature A (highest)
M+S or 3PMSF	Mud and Snow rated / Three-Peak Mountain Snowflake	3PMSF = tested for severe snow
Max PSI	Maximum inflation pressure (not recommended daily pressure)	Max Press 51 PSI

Compare Tire Sizes Side by Side

Enter any two tire sizes to compare overall diameter, width, circumference, and speedometer difference.

Open Tire Size Calculator →

Frequently Asked Questions

Where do I find the tire size on my car?

You can find your tire size in three places: on the tire sidewall itself (the most common method), on a sticker inside the driver's door jamb (which shows the manufacturer's recommended size and pressure), or in your vehicle owner's manual. The door jamb sticker is the most reliable source because it shows the size the vehicle was designed for, while the tires currently on the car may have been changed by a previous owner. On the tire sidewall, look for a sequence like P225/65R17 95H. The numbers will be prominently embossed in the rubber on the outer sidewall of every tire.

Can I put a different tire size on my car?

You can change tire sizes within certain limits, but the overall tire diameter should stay within 3% of the original to avoid speedometer inaccuracy, ABS interference, and potential clearance issues. For example, if your original tire has an overall diameter of 28.0 inches, the replacement should be between 27.2 and 28.8 inches. You can go wider for better grip or use a different aspect ratio, as long as the total diameter stays close. The Tire and Rim Association recommends matching the tire width to the wheel width within the approved range. Use the tire size calculator to compare sizes before purchasing.

What does the R mean in tire size?

The "R" in a tire size designation like P225/65R17 stands for Radial construction. In a radial tire, the internal cord plies run perpendicular (at 90 degrees) to the direction of travel, from bead to bead across the tread. Radial construction provides better fuel economy, longer tread life, improved ride comfort, and superior handling compared to older bias-ply designs. According to the Rubber Manufacturers Association, over 98% of all passenger car tires sold today are radial construction. You may occasionally see "B" for bias-belt or "D" for diagonal (bias-ply) on specialty or vintage tires.

What is the most common tire size in the US?

The most common tire size in the United States is 225/65R17, according to tire industry sales data from the Tire Industry Association. This size is standard on many mid-size SUVs and crossovers, including the Toyota RAV4, Honda CR-V, and Subaru Forester. Other popular sizes include 215/55R17 (mid-size sedans like the Honda Accord and Toyota Camry), 265/70R17 (full-size trucks like the Ford F-150), and 205/55R16 (compact cars like the Honda Civic). The trend over the past decade has been toward larger wheel diameters, with 17-inch and 18-inch rims now more common than 15-inch and 16-inch.

What happens if I use tires that are too big or too small?

Using tires that are significantly larger or smaller than the manufacturer's specification causes several problems. Tires that are too large may rub against the fender or suspension components during turns or when driving over bumps, causing damage to the tire and vehicle. They will also cause your speedometer to read lower than your actual speed. Tires that are too small will cause your speedometer to read higher than actual speed and may not have adequate load capacity for your vehicle. Both scenarios can interfere with anti-lock braking (ABS) and traction control systems. The NHTSA recommends always using tires within the size range specified by your vehicle manufacturer.

How do I calculate the overall diameter of a tire?

To calculate overall tire diameter from a size like P225/65R17, use this formula: Overall Diameter = (2 × Sidewall Height) + Rim Diameter. First, calculate the sidewall height: 225 mm × 0.65 (aspect ratio) = 146.25 mm. Convert to inches: 146.25 / 25.4 = 5.76 inches. Then: Overall Diameter = (2 × 5.76) + 17 = 28.52 inches. The circumference is diameter × π = 28.52 × 3.14159 = 89.6 inches, which determines how many tire revolutions occur per mile (approximately 708 revolutions). Use the tire size calculator to instantly compare diameters between two tire sizes.