Tire Size Calculator
Compare two tire sizes side by side. See diameter, circumference, sidewall height, speedometer difference, and revolutions per mile.
Original Tire
225/45R17
New Tire
235/40R18
| Measurement | Original | New | Difference |
|---|
Speedometer Impact
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Saved Comparisons
How to Read Tire Size Numbers
A tire size like 225/45R17 is read as follows:
225 = Section width in millimeters (tread width from sidewall to sidewall)
45 = Aspect ratio (sidewall height is 45% of the section width)
R = Radial construction (virtually all modern tires)
17 = Rim diameter in inches
Calculation Formulas
Sidewall Height: Width (mm) x (Aspect Ratio / 100)
Overall Diameter: (2 x Sidewall Height in inches) + Rim Diameter
Circumference: Overall Diameter x pi (3.14159)
Revolutions/Mile: 63,360 (inches/mile) / Circumference (inches)
Speedometer Error: (New Diameter / Old Diameter - 1) x 100%
Worked Example
Tire: 225/45R17
Step 1 - Sidewall: 225 x 0.45 = 101.25 mm = 3.99 inches
Step 2 - Diameter: (3.99 x 2) + 17 = 24.97 inches
Step 3 - Circumference: 24.97 x 3.14159 = 78.44 inches
Step 4 - Revs/Mile: 63,360 / 78.44 = 808 revolutions
Tire Upsizing Safety Tips
- Keep overall diameter change within +/-3% to avoid speedometer and ABS issues.
- Wider tires may rub against fenders or suspension components -- check clearance.
- Changing tire size can affect gear ratios, fuel economy, and ride comfort.
- Always consult your vehicle manufacturer's recommendations for approved tire sizes.
- After changing sizes, consider a speedometer recalibration or GPS verification.