Tire Size Calculator

Calculate tire dimensions from size code. Compare two tire sizes for speedometer accuracy and fitment compatibility.

First number in tire size (e.g., 225/65R17)

Second number (sidewall height % of width)

Rim size (last number)

Tire Size Format: Width/AspectRatio R WheelDiameter\n\nSidewall Height = Width * (Aspect Ratio / 100)\nTotal Diameter = Wheel Diameter + (2 * Sidewall Height)\nCircumference = PI * Total Diameter\nRevs/Mile = 63,360 / Circumference (inches)\n\nSpeedometer Error % = ((New Diameter - Old Diameter) / Old Diameter) * 100
Example 1:\nTire: 225/65R17\n\nWidth: 225 mm\nSidewall: 225 * 0.65 = 146.25 mm\nDiameter: 17 + (2 * 5.76) = 28.52 in\nCircumference: 89.58 inches\n\nExample 2 (Compare):\nOld: 225/65R17 (28.52 in)\nNew: 245/60R17 (28.57 in)\n\nDifference: +0.05 in (+0.18%)\nSpeedometer Error: +0.18%\nAt 60 mph speedo -> 60.1 mph actual\nSafe change (under 3%)

How do I read tire size numbers?

Example: P225/65R17. P = Passenger car. 225 = width in millimeters. 65 = aspect ratio (sidewall height is 65% of width). R = Radial construction. 17 = wheel diameter in inches. So sidewall height = 225mm × 0.65 = 146.25mm.

What happens if I change tire size?

Larger diameter: speedometer reads slower than actual (shows 60 when going 65), odometer under-reports. Smaller: opposite effect. More than 3% difference affects accuracy, may trigger ABS/traction control issues. Wider tires: better grip, worse fuel economy.

Can I use different tire sizes on my car?

You can use "plus sizing" (keeping overall diameter within 3%). Example: 225/65R17 → 245/60R17 (wider, lower profile, same diameter). Benefits: better handling, appearance. Downsides: harsher ride, cost. Always match all 4 tires for AWD vehicles.

What is aspect ratio and why does it matter?

Aspect ratio is sidewall height as percentage of tire width. Lower = shorter sidewall, better handling, harsher ride. Higher = taller sidewall, smoother ride, worse handling. 65 is common for sedans, 55 for sport, 45 for performance.

How do I calculate speedometer error?

Speedometer Error % = ((New Diameter - Old Diameter) / Old Diameter) × 100. If new tire is 3% larger, speedometer shows 60 when you're actually going 61.8 mph. Keep error under ±3% to avoid issues.