Tire Size Calculator
Overall diameter and circumference of a tire from its size code.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
A tire's sidewall code (like 225/45R17) packs three numbers that together set its overall diameter. Knowing that diameter matters whenever you change tire sizes, because it directly affects your speedometer reading and ride height.
How it works
Sidewall height is width × aspect ratio (in inches); diameter is rim plus two sidewalls.
The aspect ratio is a percentage of the tread width, not a fixed measurement — so a 45 on a 225 tire means the sidewall is 45% of 225mm. Two sidewalls (top and bottom) plus the rim diameter give the full wheel height. Everything is converted to inches so it lines up with the rim size.
Diameter drives two things you feel: revolutions per mile (which the speedometer counts) and effective gearing. Going taller raises your true speed above what the gauge shows and slightly lowers your effective gear ratio; going shorter does the reverse.
Sidewall height (in) = tread width × aspect ratio ÷ 25.4. Diameter = rim + 2 × sidewall. Circumference = diameter × π. Revs per mile = 63,360 ÷ circumference.
Worked examples
A 225/45R17 tire (225mm wide, 45% aspect ratio, 17-inch rim). → About 24.97 inches in diameter, with a 78.5-inch circumference.
Sidewall: 225 × 0.45 ÷ 25.4 ≈ 3.99 in. Diameter: 17 + 2 × 3.99 ≈ 24.97 in. That works out to roughly 808 revolutions per mile.
Upsizing to 245/40R18 on a wider wheel. → About 25.72 inches — roughly 3% taller.
Sidewall: 245 × 0.40 ÷ 25.4 ≈ 3.86 in. Diameter: 18 + 2 × 3.86 ≈ 25.72 in. The taller diameter would make your speedometer read about 3% slow.
Tips & gotchas
- Keep a new tire within about 3% of your stock diameter, or your speedometer and odometer will read noticeably off and traction or ABS systems may be confused.
- A taller tire makes the speedometer read slower than your true speed; a shorter one makes it read faster — handy to know when comparing GPS speed to the gauge.
- Wider isn't automatically taller — a 245/40 can be close to a 225/45 in diameter because the lower aspect ratio offsets the extra width.
- When stepping up rim size for looks, drop the aspect ratio to keep overall diameter the same (a 'plus-size' fit) and preserve your gearing and speedometer accuracy.
FAQ
What do the three numbers in 225/45R17 actually mean?
225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percent of width), R means radial construction, and 17 is the rim diameter in inches that the tire mounts to.
Will a different tire size mess up my speedometer?
It can. The speedometer is calibrated to the stock diameter, so a taller tire makes it read low and a shorter one makes it read high. Staying within ~3% keeps the error small enough to ignore.
Does a taller tire change my gearing?
Effectively yes. A larger diameter covers more ground per revolution, which lowers your effective final-drive ratio — you'll feel slightly slower acceleration and lower RPM at a given speed.
Related calculators
Gas Savings Calculator · Lease vs Buy (Car) Calculator · MPG Calculator · 0-60 mph Estimator · Towing Payload Calculator