RollingSpecTire Size Calculator
Compare a current and a new tire size to see the difference in diameter, circumference, ground clearance and speedometer reading. Enter width in millimeters, aspect ratio as a percentage, and wheel diameter in inches.
Geometry comparison only. Results are estimates based on nominal dimensions. Fitment depends on additional vehicle-specific factors — wheel-well clearance, suspension travel, load rating, and manufacturer specifications. This tool does not state whether a tire fits or is safe for your vehicle.
How to use the tire size calculator
A tire size such as 225/65R17 breaks down into three numbers: 225is the tire width in millimeters, 65 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches. Enter these three values for your current tire and the new tire you're considering to see a full geometry comparison.
How tire diameter is calculated
Sidewall height is the tire width multiplied by the aspect ratio. Overall diameter is the wheel diameter (converted to millimeters) plus two sidewalls — one above and one below the wheel. From diameter, circumference and an approximate revolutions-per-mile figure follow directly.
What happens when tire diameter changes?
A larger overall diameter increases ground clearance slightly, can change how gearing feels (engine effectively "sees" a taller final drive), and reduces revolutions per mile — which throws off a speedometer calibrated for the original tire. A smaller diameter has the opposite effects. Wheel-well and suspension clearance are not captured by this calculator and should be checked separately.
How much tire-size difference is acceptable?
There is no single universal safe percentage. Many enthusiasts use a rough diameter-difference guideline (often within a few percent) as a starting point for comparison, but this is not a standard and does not account for load rating, wheel width compatibility, vehicle electronics (ABS/traction control calibration), or physical clearance. Always check manufacturer specifications for your specific vehicle.
Tire size and speedometer error
Your speedometer is calibrated to the rolling circumference of the tire your vehicle shipped with. A larger-diameter tire covers more ground per revolution, so your actual speed will be higher than the indicated speed. A smaller-diameter tire has the opposite effect. The table above estimates actual speed at 30, 60 and 70 mph indicated.
Need only the overall dimensions of a single tire? Use theTire Diameter Calculator. Changing wheels as well as tires? See the Wheel Offset Calculator.
FAQ
How do I calculate tire diameter?
Overall diameter equals the wheel diameter plus twice the sidewall height. Sidewall height is the tire width multiplied by the aspect ratio (as a percentage). For example, a 225/65R17 has a sidewall of 225 × 0.65 = 146.25 mm, giving a diameter of (17 × 25.4) + (2 × 146.25) ≈ 724.3 mm.
What does 225/65R17 mean?
225 is the tire's width in millimeters, 65 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches that the tire is designed to fit.
Will larger tires make my speedometer inaccurate?
Yes, if the new tire has a different rolling diameter. A larger diameter means the tire covers more distance per revolution, so your actual speed will be higher than what the speedometer indicates (which is calibrated for the original tire). The comparison table above estimates this difference at common speeds.
How do I compare two tire sizes?
Enter the width, aspect ratio and wheel diameter for your current tire and the tire you're considering. The calculator computes both tires' geometry and shows the differences in width, diameter, circumference, ground clearance and speedometer reading.
Does this calculator tell me whether a tire will fit?
No. It compares nominal geometry only. Actual fitment depends on additional factors such as wheel-well clearance, suspension travel, steering angle, load rating, and manufacturer specifications for your specific vehicle.
How accurate are tire size calculations?
These figures are based on published nominal dimensions from the tire size code. Actual manufactured dimensions vary slightly by brand and model, and rolling circumference in the real world also varies with inflation pressure, load, and tread wear.