Tire Size Calculator - Compare Tire Sizes

Tire Size Calculator - Compare Tire Sizes & Speedometer Error

Compare tire sizes, calculate overall diameter, speedometer error, and circumference with our free tire size calculator. See the exact differences between tire sizes.

How to Read Tire Sizes

Tire size format: P225/45R17

• P = P-Metric (passenger car)
• 225 = Tire width in millimeters
• 45 = Aspect ratio (sidewall height as % of width)
• R = Radial construction
• 17 = Wheel diameter in inches

Tire Size Calculations

Sidewall Height:

Sidewall = Width × (Aspect Ratio / 100)
Example: 225mm × 0.45 = 101.25mm = 3.99 inches

Overall Diameter:

Diameter = (Sidewall × 2) + Wheel Diameter
Example: (3.99 × 2) + 17 = 24.98 inches

Circumference:

Circumference = Diameter × π
Example: 24.98 × 3.14159 = 78.48 inches

Revolutions per Mile:

Rev/Mile = 63,360 / Circumference
Example: 63,360 / 78.48 = 807 revolutions

Speedometer Error Calculation

When you change tire sizes, your speedometer may become inaccurate because it's calibrated for a specific tire diameter.

Formula:

Error % = ((New Diameter - Old Diameter) / Old Diameter) × 100

Example:

Old: 225/45R17 = 24.98" diameter
New: 245/40R18 = 25.72" diameter

Error = ((25.72 - 24.98) / 24.98) × 100
Error = +2.96%

Speedometer reads 2.96% SLOWER than actual speed
At 60 mph indicated, actual speed is 61.8 mph

Common Tire Size Comparisons

Popular Plus-One Upgrades:

Original	Plus One	Diameter Change
205/55R16	225/45R17	-0.2%
225/45R17	245/40R18	-0.1%
235/45R17	255/40R18	+0.3%
245/45R18	265/40R19	-0.2%

Plus Sizing Explained

Plus Sizing means increasing wheel diameter while decreasing tire aspect ratio to maintain overall diameter.

• Plus Zero: Wider tire, same wheel (e.g., 195/65R15 → 205/60R15)
• Plus One: 1" larger wheel, lower profile tire (e.g., 225/45R17 → 245/40R18)
• Plus Two: 2" larger wheel, even lower profile (e.g., 225/45R17 → 255/35R19)

Benefits of Plus Sizing:

• Improved handling and cornering
• Better steering response
• Enhanced appearance
• Less sidewall flex

Drawbacks:

• Harsher ride quality
• More susceptibility to wheel damage
• Potentially more expensive tires

Tire Size Compatibility Guidelines

Recommended diameter change: ±2.5% maximum

• Within ±1%: Excellent - No noticeable issues
• ±1% to ±2.5%: Acceptable - Minor speedometer variance
• Above ±2.5%: Not recommended - Significant issues

Potential Issues with Incorrect Sizing:

• Inaccurate speedometer/odometer readings
• ABS and traction control malfunction
• Tires rubbing on fenders or suspension
• Altered vehicle handling and stability
• Possible warranty or insurance issues

Winter Tire Sizing

Narrower is better for winter:

• Narrow tires cut through snow to pavement
• Better traction in winter conditions
• Example: 225/45R18 summer → 205/55R17 winter

Winter Plus-Zero Approach:

• Go down 1-2 wheel sizes for winter
• Increase sidewall height for pothole protection
• Reduce width for better snow traction
• Example: 19" wheels → 17" winter package

Speedometer Calibration

After changing tire sizes significantly:

• Professional recalibration: Dealership or shop
• Programmers/tuners: Some vehicles can be reprogrammed
• Speedometer gear: Older vehicles with mechanical speedos
• Acknowledge the error: Remember your speedometer is off