Tire Size Converter
Convert a metric tire size (width, aspect ratio, and wheel diameter) into overall tire diameter in inches. Use this when comparing tire sizes, checking fitment, or calculating speedometer error after a tire change.
About this calculator
Tire sizes are expressed in a metric format such as 225/55R17, where 225 is the tread width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches. The overall tire diameter combines the wheel diameter with both sidewalls. The formula is: diameter (in) = wheel_diameter + (2 × tire_width × aspect_ratio / 100 / 25.4). Here, tire_width × aspect_ratio / 100 gives the sidewall height in millimeters. Dividing by 25.4 converts it to inches. Multiplying by 2 accounts for both the top and bottom sidewalls. Adding the wheel diameter gives the complete outer diameter of the mounted tire.
How to use
Take a tire labeled 225/55R17: tire width = 225 mm, aspect ratio = 55%, wheel diameter = 17 inches. Step 1 — sidewall height in mm: 225 × 55 / 100 = 123.75 mm. Step 2 — convert to inches: 123.75 / 25.4 = 4.87 inches. Step 3 — both sidewalls: 4.87 × 2 = 9.74 inches. Step 4 — add wheel diameter: 9.74 + 17 = 26.74 inches total diameter. This means the mounted tire has an overall diameter of approximately 26.74 inches.
Frequently asked questions
How do I read a metric tire size to use in this calculator?
A metric tire size follows the format Width/AspectRatioRWheelDiameter — for example, 205/65R15. The first number (205) is the tread width in millimeters. The second number (65) is the aspect ratio, representing the sidewall height as a percentage of the tread width. The letter R indicates radial construction. The last number (15) is the wheel (rim) diameter in inches. Enter these three values — 205, 65, and 15 — into the corresponding fields of the calculator.
Why does overall tire diameter matter when changing tire sizes?
Overall diameter directly affects speedometer accuracy, odometer readings, ground clearance, and whether the tire will physically fit in your wheel well without rubbing. Even a small change in diameter — say, from 26 to 28 inches — causes a speedometer error of roughly 7.7%, meaning your car travels faster than the gauge indicates. It also changes the effective gear ratio, affecting acceleration and fuel economy. Staying within about 3% of the original tire's overall diameter is a common guideline for maintaining acceptable accuracy and fitment.
What is the difference between wheel diameter and tire diameter?
Wheel diameter refers to the size of the metal rim that the tire mounts onto, measured in inches across the inner bead seat. Tire diameter is the total outer diameter of the inflated, mounted tire including both sidewalls. For a 225/55R17 tire, the wheel is 17 inches but the full tire diameter is approximately 26.7 inches — nearly 10 inches larger due to the two sidewalls. When shopping for tires, the wheel diameter must match your rim size exactly, while the overall tire diameter determines ground clearance, fitment, and speedometer calibration.