Tire Size Calculator

How to Use This Calculator

1. Enter your Tire Width (e.g. 225), Aspect Ratio (e.g. 65), and Rim Diameter (e.g. 17) from the sidewall markings.
2. Instantly see Overall Diameter, Circumference, Revolutions per Mile, and Sidewall Height.
3. Use the Compare Two Tires tab to check speedometer error when changing sizes.

Formula

Example

Frequently Asked Questions

• How do I read the numbers on my tire? ▼
  A tire size like 225/65R17 breaks down into three key measurements. The first number, 225, is the section width in millimeters — the widest point of the tire from sidewall to sidewall. The second number, 65, is the aspect ratio — the sidewall height expressed as a percentage of the width. So a 225mm wide tire with a 65 aspect ratio has a sidewall height of 225 × 0.65 = 146.25mm. The letter R indicates radial construction, which is standard for all modern passenger and light truck tires. The final number, 17, is the rim diameter in inches that the tire is designed to fit. You can then calculate the overall diameter: rim diameter plus twice the sidewall height (converted to inches). Additionally, you will find load index and speed rating letters, such as 95H, which indicate the maximum load the tire can carry and its maximum speed rating.
• Will changing tire size affect my speedometer? ▼
  Yes, significantly. Your speedometer calculates speed based on wheel rotation speed, which is directly tied to how far the tire travels per revolution — its circumference. If you install a tire with a larger overall diameter, it covers more ground per revolution, so your actual speed will be higher than what the speedometer reads. Conversely, a smaller diameter tire makes the speedometer over-read your actual speed. The error is proportional: a 3% larger tire means your speedometer reads 60 mph when you are actually going about 61.8 mph. Most jurisdictions allow up to a 3% speedometer deviation, so keeping tire diameter within roughly 1 inch of stock is the standard guideline. This also affects your odometer — a larger tire means fewer revolutions per mile, so your recorded mileage will be lower than actual distance traveled. Use the Compare Two Tires tab to calculate the exact speedometer error percentage before making a fitment change.
• What is the difference between H, V, and W speed ratings? ▼
  Speed ratings indicate the maximum sustained speed a tire is designed to handle safely under its rated load. The rating is stamped on the sidewall as part of the service description — for example, 95H means load index 95 with an H speed rating. H-rated tires are certified to 130 mph (210 km/h) and are common on family sedans and crossovers. V-rated tires are certified to 149 mph (240 km/h) and appear on sport sedans and entry performance vehicles. W-rated tires handle 168 mph (270 km/h) and are used on sports cars and high-performance sedans. Y-rated tires are certified to 186 mph (300 km/h) — typically found on supercars and track-focused vehicles. It is important to match or exceed your vehicle manufacturer's minimum speed rating. Installing a lower-rated tire than specified can compromise handling at high speed and may void insurance coverage in an accident. You do not need to drive at those speeds for the rating to matter — higher-rated tires also generally have stiffer compounds and better high-speed heat management.
• How does tire size affect fuel economy? ▼
  Tire size affects fuel economy through rolling resistance, weight, and aerodynamic drag. Wider tires have more contact patch area, which increases rolling resistance and reduces fuel efficiency — typically 1–2% per significant width increase. Taller sidewalls (higher aspect ratio) flex more during driving, which also increases rolling resistance and heat generation. However, a larger overall tire diameter means fewer wheel revolutions per mile, which can slightly improve highway efficiency for the same engine RPM. Tire weight matters too: heavier tires require more energy to accelerate and decelerate — unsprung rotating mass has a multiplied effect on efficiency. Low rolling resistance tires with fuel-economy-optimized compounds are labeled with the tire industry UTQG (Uniform Tire Quality Grading) and can improve MPG by 1–4% compared to standard tires. The EPA tests fuel economy on OEM tire specifications, so significant deviations can reduce real-world MPG.
• Can I use winter tires year-round? ▼
  Technically yes, but it is strongly inadvisable. Winter tires are engineered with a special soft rubber compound containing a higher silica content that stays pliable at temperatures below 45°F (7°C), providing superior grip on snow and ice. Above that temperature threshold, the same soft compound becomes too flexible, which causes accelerated tread wear, increased rolling resistance, reduced cornering stability, and longer braking distances on dry pavement. A set of winter tires used year-round will typically wear out in one to two seasons rather than the usual three to four winters of seasonal use. They are also more expensive to replace. Additionally, many winter tires have an aggressive tread pattern optimized to channel slush and bite into snow — this increases road noise and reduces hydroplaning resistance on warm wet roads compared to all-season tires. The cost of owning two sets of tires (including a second set of wheels to avoid mounting fees) is almost always recouped in longer tire lifespan and improved safety.

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