Complete guide
Tire size calculator and side-by-side comparison
Enter the width, aspect ratio and wheel diameter printed in each metric tire size. Calculator24 compares sidewall, overall diameter, circumference, revolutions per mile, ground-clearance change and estimated actual speed.
Read a metric tire size
In 225/65R17, 225 is nominal section width in millimeters, 65 is sidewall height as a percentage of width, R identifies radial construction and 17 is wheel diameter in inches. Enter the three numbers for both sizes.
Diameter and circumference formulas
Convert sidewall millimeters to inches, double it and add the wheel diameter. Circumference uses π times overall diameter; revolutions per mile divide 63,360 inches by circumference.
sidewall (in) = width × aspect ÷ 100 ÷ 25.4diameter = rim + 2 × sidewallcircumference = π × diameterSpeedometer difference
A larger tire travels farther per revolution. The estimated actual speed equals indicated speed multiplied by the new-to-current diameter ratio. Vehicle calibration, wear and pressure can change the real reading.
Clearance and fitment
Half the diameter difference estimates the change in axle height and static ground clearance. Width, offset, suspension travel, steering angle and tire shape determine actual rubbing clearance and are outside this geometry model.
Safety before changing sizes
NHTSA says to use the original tire size or another size recommended by the vehicle manufacturer and to check the door label or owner's manual. A close diameter match does not prove compatible load capacity, speed rating, rim width or inflation pressure.
Metric tire code
| Part | Example | Meaning |
|---|---|---|
| Width | 225 | Nominal millimeters |
| Aspect ratio | 65 | Sidewall is 65% of width |
| Construction | R | Radial |
| Wheel | 17 | Wheel diameter in inches |
Frequently asked questions
How is tire diameter calculated?
Add the wheel diameter to twice the sidewall height after converting millimeters to inches.
What does 225/65R17 mean?
It denotes 225 mm width, 65% aspect ratio, radial construction and a 17-inch wheel.
How does tire size affect the speedometer?
Diameter changes distance traveled per wheel revolution, so actual speed changes in the same ratio.
What diameter difference is safe?
This calculator does not approve fitment; follow the vehicle manufacturer or a qualified tire professional.
Does matching diameter guarantee fit?
No. Width, rim, offset, load rating, suspension and body clearance also matter.
What to keep in mind
Geometry only. Confirm approved size, load rating, speed rating, wheel fitment, clearance and inflation pressure with the vehicle and tire manufacturers.
Results display up to 8 decimal places; exported numbers preserve calculation precision. This is a calculation summary, not an official certificate.