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Tire Size Calculator Guide
Thinking of changing tire sizes? This calculator converts a tire size like 225/45R17 into its real overall diameter, sidewall height, and revolutions per mile — and compares two sizes so you can see the fit difference and how it affects your speedometer.
Sidewall = width × aspect ratio. Overall diameter = wheel diameter + 2 × sidewall. A 225/45R17 has a sidewall of 225 × 0.45 = 101 mm and a diameter of about 25 inches.
How Do I Read a Tire Size?
In "225/45R17": 225 is the tread width in millimetres, 45 is the aspect ratio (sidewall height as a percent of width), R means radial, and 17 is the wheel diameter in inches. From those three numbers the overall diameter follows directly.
Why Diameter Matters
- Speedometer accuracy: a bigger diameter makes your speedo read slow (you're actually going faster than it shows).
- Clearance: too large a tire can rub the fender or suspension.
- Gearing feel: larger tires slightly reduce acceleration and lower RPM at speed.
A safe swap usually keeps the overall diameter within about 3% of the original so the speedometer and systems stay accurate.
Worked Example
225/45R17: sidewall = 225 × 0.45 = 101.25 mm; diameter = 17 × 25.4 + 2 × 101.25 = 634.3 mm ≈ 24.97 in, about 808 revolutions per mile.
Bigger tires can slightly change your real MPG reading — check it with the Gas Mileage Calculator, and budget fuel with the Fuel Cost Calculator.
Frequently Asked Questions
How do I read tire size?
In 225/45R17, 225 is the width in mm, 45 is the sidewall as a percent of width, R is radial, and 17 is the wheel diameter in inches.
How do I calculate tire diameter?
Multiply the width by the aspect ratio for the sidewall, then add twice the sidewall to the wheel diameter.
How much bigger tire can I fit?
As a rule of thumb, keep the overall diameter within about 3% of the original to avoid rubbing and speedometer error.
Do bigger tires affect the speedometer?
Yes — a larger diameter makes the speedometer read slower than your actual speed, and a smaller one reads faster.
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