Tire Size Calculator
Free Tire Size Calculator for automotive. Free online tool with accurate results using verified formulas.
Formula
Diameter = Rim (in mm) + 2 ร (Width ร Aspect Ratio / 100)
Calculate overall tire diameter by adding the rim diameter (converted to mm) to twice the sidewall height. Sidewall height is the tire width multiplied by the aspect ratio percentage.
Worked Examples
Example 1: Understanding Tire Size Code
Problem: Decode the tire size 245/40R18 and calculate its dimensions.
Solution: Width = 245 mm\nAspect Ratio = 40% of width\nRim Diameter = 18 inches\n\nSidewall Height:\n245 ร 0.40 = 98 mm\n\nOverall Diameter:\nRim in mm = 18 ร 25.4 = 457.2 mm\nDiameter = 457.2 + (2 ร 98) = 653.2 mm\n653.2 รท 25.4 = 25.72 inches\n\nCircumference:\n653.2 ร ฯ = 2052 mm = 80.8 inches
Result: 245/40R18 = 25.7\" diameter, 98mm sidewall
Example 2: Plus-One Sizing Comparison
Problem: Compare original 205/55R16 with plus-one upgrade 225/45R17.
Solution: Original 205/55R16:\nSidewall = 205 ร 0.55 = 112.75 mm\nDiameter = (16 ร 25.4) + (2 ร 112.75) = 632.1 mm = 24.89\"\n\nUpgrade 225/45R17:\nSidewall = 225 ร 0.45 = 101.25 mm\nDiameter = (17 ร 25.4) + (2 ร 101.25) = 634.3 mm = 24.97\"\n\nDifference: 24.97 - 24.89 = 0.08\" (0.3%)\n\nThis is an ideal plus-one setup - nearly identical diameter!
Result: Only 0.3% diameter difference - perfect plus-one sizing
Example 3: Speedometer Error Calculation
Problem: Original tires are 265/70R17. New tires are 285/70R17. What's the speedometer error?
Solution: Original 265/70R17:\nSidewall = 265 ร 0.70 = 185.5 mm\nDiameter = (17 ร 25.4) + (2 ร 185.5) = 802.8 mm = 31.61\"\n\n New 285/70R17:\nSidewall = 285 ร 0.70 = 199.5 mm\nDiameter = (17 ร 25.4) + (2 ร 199.5) = 830.8 mm = 32.71\"\n\nSize increase: (32.71 - 31.61) / 31.61 ร 100 = 3.5%\n\nAt 60 mph indicated: Actual = 60 ร 1.035 = 62.1 mph
Result: 3.5% larger - speedo reads 2 mph slow at 60
Frequently Asked Questions
What do tire numbers like 225/45R17 mean?
The tire size code breaks down as: 225 = width in millimeters (section width from sidewall to sidewall). 45 = aspect ratio (sidewall height as percentage of width; 45% of 225mm = 101.25mm sidewall). R = radial construction (nearly all modern tires). 17 = rim diameter in inches. So 225/45R17 means 225mm wide, with sidewalls 45% of that height (101mm), for a 17-inch wheel.
How do I know if a different tire size will fit my car?
Generally, stay within 3% of the original tire diameter to avoid speedometer errors, clearance issues, and potential transmission/ABS problems. You can go wider if your wheels and fenders can accommodate it. Check: fender clearance (especially when turning/suspension compressed), wheel width compatibility (tire width should be within 0.5-1 inch of wheel width), and load/speed rating requirements. Tire shops and online fitment guides can help confirm.
How does tire size affect speedometer accuracy?
Larger tires make your speedometer read SLOW (you're going faster than indicated). Smaller tires make it read FAST (you're going slower). A 3% larger tire means at an indicated 60 mph, you're actually going 61.8 mph. The formula: Actual Speed = Indicated Speed ร (New Diameter / Old Diameter). Most vehicles can tolerate 1-3% variance. Beyond that, speedometer recalibration may be needed.
What does tire width affect?
Wider tires provide: Larger contact patch for better grip (acceleration, cornering, braking), sportier appearance, but increased road noise, hydroplaning risk, and fuel consumption. Narrower tires: Better fuel economy, better performance in snow (concentrate weight for better penetration), lighter weight. Width should match wheel width - too narrow or wide causes handling issues and uneven wear.
Can I mix tire sizes on my car?
Staggered setups (wider rear tires) are factory-installed on many sports cars (RWD) for rear traction. However: All four tires should have the same overall diameter on AWD vehicles (different sizes can damage the differential). On RWD/FWD, different axle sizes are acceptable if designed for it. Never mix tires on the same axle. Snow tires should be the same size all around. Check your owner's manual for specific recommendations.
How do I read the other numbers on a tire sidewall?
Beyond the size, you'll find: Load Index (91 = 1356 lbs max per tire), Speed Rating (H = 130 mph max), DOT code (manufacturing date in WWYY format, e.g., 2523 = week 25 of 2023), Treadwear/Traction/Temperature ratings (UTQG), maximum inflation pressure, and the manufacturer's name/model. The load index and speed rating must meet or exceed original tire specs for safety.