Tire Size Calculator
Decode tire sizes like 205/55R16: get diameter, sidewall height, circumference, and revs per mile — plus speedometer error when comparing two sizes.
Upgrading wheels or replacing tires? Enter any tire size code to see its true diameter, sidewall height, circumference, and revolutions per mile. Enter a second size to compare: the calculator shows the percentage difference and how far off your speedometer will read, so you can stay within the safe 3% window.
Decode a tire size like 205/55R16: 205 mm wide, sidewall 55% of the width, 16-inch rim. Enter two sizes to compare.
Current tire
Replacement tire
How to use this calculator
- Read the size code from your current tire's sidewall (e.g. 205/55R16).
- Enter width, aspect ratio, and rim diameter under 'Current tire'.
- Enter the size you're considering under 'Replacement tire'.
- Press Calculate to see both specs, the diameter difference, and the speedometer effect.
The formula
Sidewall = width × aspect ratio ÷ 100; diameter = rim + 2 × sidewall
Worked example
A 205/55R16 measures 24.88 inches in diameter. Swapping to a 225/45R17 is only 0.38% larger — a safe swap — and your speedometer would read 59.77 mph at a true 60 mph.
Frequently Asked Questions
What do the numbers in 205/55R16 mean?
205 is the tire's width in millimeters, 55 is the aspect ratio — the sidewall height as a percentage of the width — and 16 is the wheel (rim) diameter in inches. The R stands for radial construction, which virtually all modern car tires use. So a 205/55R16 is 205 mm wide with a sidewall 55% of that width, fitting a 16-inch wheel.
How much bigger can I go before it causes problems?
The industry rule of thumb is to stay within 3% of the original overall diameter. Beyond that, your speedometer and odometer readings drift, gearing changes, and the tire may rub the wheel wells or suspension. The comparison tool flags anything over 3% as risky — always confirm clearance and load rating with a tire professional.
Will bigger tires make my speedometer wrong?
Yes. Your speedometer counts wheel revolutions and assumes the factory tire size. A larger-diameter tire covers more ground per revolution, so the speedometer reads lower than your actual speed — with a 3% larger tire, an indicated 60 mph is really about 61.8 mph. The comparison table shows exactly what your speedometer will read at a true 60 mph.
Does changing tire size affect gas mileage?
Usually yes, slightly. Wider or larger tires are heavier and have more rolling resistance, which can cost 1–3 MPG. Low-profile tires on larger wheels are also typically heavier. If fuel economy matters to you, staying close to the factory size and choosing a low-rolling-resistance tire is the best move.
What are revs per mile used for?
Revolutions per mile tells you how many times the tire rotates over one mile — it's the number your car's computer uses for speed, odometer, ABS, and traction control. When comparing sizes, matching revs per mile within a few percent keeps all those systems behaving as designed.