Tire size calculator and side-by-side comparison
Enter the width, aspect ratio and wheel diameter printed in each metric tire size. HitEquals compares sidewall, overall diameter, circumference, revolutions per mile, ground-clearance change and estimated actual speed.
Inputs used by this calculator
Tire Size Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Current tire width (mm) | 225 | 50 to 500 | Supplies the current tire width (mm) quantity used by the published equation or result breakdown. |
| Current aspect ratio (%) | 65 | 10 to 100 | Supplies the current aspect ratio (%) quantity used by the published equation or result breakdown. |
| Current wheel diameter (in) | 17 | 8 to 30 | Supplies the current wheel diameter (in) quantity used by the published equation or result breakdown. |
| New tire width (mm) | 235 | 50 to 500 | Supplies the new tire width (mm) quantity used by the published equation or result breakdown. |
| New aspect ratio (%) | 60 | 10 to 100 | Supplies the new aspect ratio (%) quantity used by the published equation or result breakdown. |
| New wheel diameter (in) | 18 | 8 to 30 | Supplies the new wheel diameter (in) quantity used by the published equation or result breakdown. |
| Indicated speed (mph) | 60 | 0 to 300 | Supplies the indicated speed (mph) quantity used by the published equation or result breakdown. |
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.
What the Tire Size Calculator answers
Compare two metric tire sizes, diameter, circumference, revolutions and speedometer change.
Use Tire Size Calculator when you know Current tire width (mm), Current aspect ratio (%), Current wheel diameter (in), New tire width (mm), New aspect ratio (%), New wheel diameter (in) and Indicated speed (mph) and need the relationship expressed by the published equation.
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 question does this Tire Size Calculator answer?
Compare two metric tire sizes, diameter, circumference, revolutions and speedometer change.
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 are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.