Tire Size Calculator
The short answer: a tire size is three numbers — width in millimeters, sidewall as a percentage of that width, and wheel diameter in inches — and from those, geometry gives you everything else: sidewall height, overall diameter, circumference, and revolutions per mile. Add a second size to see the diameter difference and what it does to your speedometer.
New rubber is the cheap insurance
Tires are the one part touching the road — and worn or mismatched sets are a safety item, not a cosmetic one. Extended warranty and service-plan partners are the planned partners for this slot; for tires themselves, buy on the load/speed rating your door-jamb placard specifies, not just the size.
How this is calculated
Sidewall height = width × aspect ratio ÷ 100 (millimeters). Overall diameter = wheel diameter × 25.4 + 2 × sidewall. Circumference = π × diameter. Revolutions per mile = 63,360 inches in a mile ÷ circumference. Verify with the defaults: a 225/65R17 has a 146.25 mm sidewall, a 28.52-inch diameter, and about 707 revolutions per mile — the same figures tire manufacturers publish. The speedometer comparison assumes the car was calibrated for the first size: true speed scales with the diameter ratio. Note that a tire's loaded rolling diameter is slightly smaller than the free diameter computed here, which is why manufacturers' rev-per-mile figures can differ by a percent or so.
Frequently asked questions
How do I read a tire size like 225/65R17?
225 is the tread width in millimeters. 65 is the aspect ratio — the sidewall height as a percentage of the width (so this sidewall is 65% of 225 mm ≈ 146 mm). R means radial construction, and 17 is the wheel diameter in inches. Those three numbers are everything this calculator needs.
How much size change is safe when swapping tires?
The common rule of thumb is to stay within about ±3% of the original overall diameter. Beyond that, the speedometer and odometer drift noticeably, gearing and driver-assist calibration change, and on all-wheel-drive cars a mismatched diameter can strain the drivetrain. The comparison mode above shows your exact percentage.
Why does my speedometer read differently with new tires?
The speedometer counts wheel revolutions and assumes a stock tire circumference. A larger-diameter tire covers more ground per revolution, so you travel faster than the dial shows (and accumulate fewer odometer miles than real ones). This tool converts that directly: at an indicated 60 mph it shows your true speed on the second tire size.
What are revolutions per mile used for?
Gearing and calibration work: matching tire changes to speedometer correction, comparing effective gearing between setups, and checking manufacturer specs — many tire makers publish revs per mile for each size, so it's also a good cross-check of this calculator (a 225/65R17 comes out near 707).