🛞 Tire Size Calculator
Enter a metric tire size to see its sidewall height, overall diameter, circumference, and revolutions per mile — and add a second size to check the speedometer error of a plus-size swap.
📐 Width, Aspect Ratio & Rim
What is a Tire Size Calculator?
It translates the three numbers stamped on a tire's sidewall — width, aspect ratio, and rim diameter — into the dimensions that actually affect your car: how tall the tire stands, how far it rolls per turn, and how many times it spins per mile. Those figures drive your speedometer reading, effective gearing, and whether a new size will clear the fenders.
Comparing two sizes side by side shows the speedometer error a swap introduces, so you can plus-size wheels or fit larger rubber with eyes open. The numbers are estimates for planning — confirm real fitment against the manufacturer's spec.
❓ Frequently Asked Questions
How do I read a tire size like 225/45R17?
The first number (225) is the section width in millimetres. The second (45) is the aspect ratio — the sidewall height as a percentage of the width, so here 45% of 225 mm. The R means radial construction, and the last number (17) is the wheel diameter in inches the tire is built to fit. This calculator turns those three figures into the tire's real-world dimensions.
What is speedometer error and why does it matter?
Your speedometer is calibrated to the original tire's rolling diameter. Fit a taller tire and each revolution covers more ground, so your true speed is higher than the gauge shows; fit a shorter one and it reads high. The comparison mode reports the percentage difference in diameter between two sizes — roughly the percentage your speedometer and odometer will be off.
What does 'revolutions per mile' tell me?
It's how many times the tire turns to cover one mile, derived from its circumference. A larger tire turns fewer times per mile, which subtly changes effective gearing — it lowers engine RPM at a given speed and shifts your speedometer reading. Comparing revs per mile between two sizes is a quick way to gauge how a swap affects gearing.
How accurate is the calculated diameter?
The geometry is exact for the numbers you enter, but these are estimates for planning; actual rolling diameter varies by brand, tread design and wear, inflation pressure, and load. Always confirm fitment and load/speed ratings against the manufacturer's spec before buying.