Tire Size Calculator

Understand and Compare Any Tire Size

Tire codes like "225/65R17 102H" pack a surprising amount of information into a cryptic format. Our calculator decodes any tire size into actual dimensions — diameter, width, sidewall height, circumference, and revolutions per mile — and compares two sizes side-by-side to show differences in height, width, and the critical speedometer accuracy impact of changing sizes.

Enter your current tire size and a potential replacement size. The calculator displays both tires' complete specifications and highlights the percentage differences that affect your vehicle's behavior, fuel economy, and instrument accuracy.

Decoding example — 225/65R17 102H: 225 = section width in millimeters (8.86 inches). 65 = aspect ratio (sidewall height is 65% of width). R = radial construction. 17 = wheel diameter in inches. 102 = load index (1,874 lbs per tire). H = speed rating (130 mph maximum). Overall diameter: approximately 28.5 inches.

Breaking Down the Tire Size Code

Section Width (First Number)

The section width is the tire's width from sidewall to sidewall in millimeters, measured when mounted on the recommended wheel width and inflated to the specified pressure. A 225mm tire is approximately 8.86 inches wide. Common passenger car widths range from 185mm (compact cars) to 275mm (performance and luxury vehicles). Truck and SUV tires range from 245mm to 325mm+.

Width affects performance: Wider tires provide a larger contact patch with the road, improving grip during acceleration, braking, and cornering. The trade-off: wider tires have more rolling resistance (reducing fuel economy by 1–3%), generate more road noise, are more susceptible to hydroplaning (less pressure per square inch pushes water aside less effectively), and cost more. A 255mm tire costs approximately 15–25% more than a 225mm tire in the same brand and model.

Width must match your wheel: Each wheel width has a range of compatible tire widths. A 7-inch wheel accommodates 205–235mm tires. An 8-inch wheel fits 225–255mm. Mounting a tire too wide or too narrow for the wheel compromises handling, ride quality, and safety.

Aspect Ratio (Second Number)

The aspect ratio is the sidewall height expressed as a percentage of the section width. A 65 aspect ratio on a 225mm tire means the sidewall is 225 × 0.65 = 146.25mm (5.76 inches) tall. This measurement from the wheel rim to the tread surface determines ride comfort, handling feel, and vulnerability to pothole damage.

Lower aspect ratios (30–45): Short, stiff sidewalls. Found on sports cars and performance vehicles. Advantages: precise steering response, minimal sidewall flex during cornering, aggressive appearance. Disadvantages: harsh ride, increased sensitivity to potholes and road imperfections, higher risk of wheel damage from impacts.

Medium aspect ratios (50–65): The most common range for passenger cars and crossovers. Balanced ride comfort and handling. A 55 or 60 aspect ratio is the sweet spot most drivers find comfortable.

Higher aspect ratios (70–85): Tall, cushioning sidewalls. Found on trucks, SUVs, and older vehicles. Advantages: comfortable ride, excellent pothole absorption, good off-road capability. Disadvantages: less precise handling feel, more body roll during cornering, "mushy" steering feedback.

Construction Type and Wheel Diameter

R = Radial construction — the universal standard for modern passenger vehicles. Radial tires have cord plies running perpendicular to the direction of travel, providing better heat dissipation, fuel economy, and tread life than the older bias-ply construction. Nearly 100% of passenger car tires sold today are radial.

The wheel diameter (17 in our example) is the inner diameter of the tire that matches the wheel's outer diameter. This must match exactly — a tire designed for a 17-inch wheel cannot be mounted on a 16-inch or 18-inch wheel. Common diameters: 15" (economy and older vehicles), 16"–17" (most sedans, compacts, and small SUVs), 18"–20" (mid-size SUVs, trucks, and performance vehicles), 20"+ (full-size trucks, luxury vehicles).

Load Index and Speed Rating

Load index (102 in our example) indicates the maximum weight each tire can support when properly inflated. 102 = 1,874 lbs per tire, or 7,496 lbs total for four tires. Never install tires with a load index lower than your vehicle manufacturer's specification — doing so creates a safety hazard.

Speed rating (H in our example) indicates the maximum sustained speed the tire is designed to handle. H = 130 mph. T = 118 mph. V = 149 mph. W = 168 mph. Y = 186 mph. Z = 149+ mph (the exact limit is determined by the manufacturer). Your tire's speed rating should meet or exceed your vehicle's top speed capability. Speed ratings also reflect handling capability — higher-rated tires generally have better grip and stability at highway speeds.

What Happens When You Change Tire Size

Changing tire size affects multiple vehicle systems. Our calculator quantifies each impact so you can make an informed decision.

Speedometer accuracy is directly affected by overall tire diameter. Your speedometer is calibrated for the original tire's circumference — each revolution of the tire is assumed to cover a specific distance. A larger tire covers more distance per revolution, making your speedometer read low (displaying a slower speed than actual). A smaller tire reads high (displaying faster than actual).

The formula: Speedometer error = (new diameter ÷ original diameter - 1) × 100%. A change from 28.0" to 29.0" overall diameter: (29.0 ÷ 28.0 - 1) × 100% = +3.57%. At an indicated 60 mph, you're actually traveling 62.1 mph. At 70 mph indicated, actual speed is 72.5 mph. This matters for speed limits, fuel economy calculations, and odometer accuracy.

Odometer accuracy follows the same math. If your tires are 3.5% larger than original, your odometer records 3.5% fewer miles than you actually travel. Over 100,000 miles, that's 3,500 "missing" miles — which affects lease mileage tracking, resale value claims, and maintenance interval accuracy.

Ground clearance increases with larger overall diameter and decreases with smaller. Each inch of additional tire diameter adds approximately 0.5 inches of ground clearance (because only the bottom half of the tire is below the axle). For off-road vehicles, this can be significant.

Fuel economy is affected by both tire width and diameter. Wider tires increase rolling resistance (lower MPG). Taller tires change the effective gear ratio — similar to adding a taller final drive gear — which can slightly improve highway MPG but reduce low-speed acceleration. Net effect: most tire size changes that increase width or diameter reduce fuel economy by 1–4%.

Acceleration and braking are subtly affected. Larger diameter tires effectively lengthen your gear ratios, reducing acceleration (the engine has to work harder per wheel revolution). Braking distance can increase slightly with larger tires if the ABS system isn't recalibrated. Wider tires generally improve braking grip but the diameter effect works against this.

Plus Sizing and Minus Sizing

Plus sizing increases wheel diameter while decreasing sidewall height to maintain approximately the same overall tire diameter. Example: going from 225/65R17 to 245/50R18. The wheel is 1 inch larger, the tire is wider with a shorter sidewall, but the overall diameter is similar. Benefits: improved handling response (less sidewall flex), more aggressive appearance, wider contact patch. Trade-offs: harsher ride, increased cost (bigger wheels + wider tires), and more vulnerability to pothole damage.

Plus-zero sizing changes the tire width and aspect ratio without changing wheel diameter. Example: 205/55R16 to 225/45R16 — wider tire, shorter sidewall, same 16-inch wheel. This maintains your existing wheels while changing tire performance characteristics.

Minus sizing does the opposite — smaller wheels, taller sidewalls. Example: going from 18-inch to 17-inch wheels with corresponding tire adjustment. Benefits: softer ride, less expensive tires and wheels, better pothole resistance. Common for winter tire setups where comfort and durability matter more than appearance.

The critical rule for any size change: Stay within 3% of your original overall tire diameter. This range keeps speedometer error, ABS calibration, traction control behavior, and transmission shift points within acceptable tolerances. Our calculator shows the exact percentage difference and warns you when you exceed 3%.

Reading the Tire Sidewall: Date Codes and Other Markings

Beyond the size code, your tire sidewall contains additional critical information.

DOT date code (last four digits of the DOT number): The first two digits are the manufacturing week, the last two are the year. "3524" means the tire was made in the 35th week of 2024. Tires older than 6 years should be inspected by a tire professional. Tires older than 10 years should be replaced regardless of tread depth — rubber compounds degrade with age, losing flexibility and grip.

Treadwear rating (e.g., 500, 700): A relative durability index. A tire rated 400 should last twice as long as a tire rated 200 under standardized test conditions. Higher numbers = longer tread life. Performance tires typically rate 200–400. All-season touring tires: 500–800.

Traction rating (AA, A, B, C): Measures straight-line wet braking grip. AA is the highest. Most passenger tires are A or AA.

Temperature rating (A, B, C): Measures heat resistance at sustained high speeds. A is the highest. Performance tires are typically A; budget tires may be B or C.

M+S (Mud and Snow): Indicates the tread pattern provides some traction in mud and light snow. This is a design-based rating, not a performance test. For serious winter driving, look for the Three-Peak Mountain Snowflake (3PMSF) symbol — this indicates the tire passed actual snow traction testing.