Tire Size Calculator
Compare two metric tire sizes for diameter, circumference, percentage difference, and speedometer effect.
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How this calculator works
Overall diameter in inches = wheel diameter in inches + 2 × (section width in mm × aspect ratio ÷ 100 ÷ 25.4). Enter the aspect ratio as the percentage printed on the tire, such as 65, not 0.65. Speed changes in proportion to the new-to-original diameter ratio.
Worked example
Comparing 225/65R17 with 235/60R18 shows the overall diameter change and the estimated actual speed at an indicated 60 mph.
Read a metric tire size correctly
In a size such as 225/65R17, 225 is section width in millimeters, 65 is sidewall height as a percentage of width, and 17 is wheel diameter in inches. Overall diameter combines two sidewalls and the wheel. Actual mounted dimensions can vary by tire model and approved rim width.
Compare both diameter and load requirements. A similar diameter does not guarantee adequate load capacity, wheel compatibility, clearance, or safe performance.
What a percentage difference means
A larger replacement travels farther per revolution, so the indicated speed can be lower than actual speed; a smaller tire does the opposite. Diameter changes can also affect gearing, odometer readings, driver-assistance systems, spare-tire compatibility, and clearance. Confirm alternatives with the vehicle and tire manufacturers or a qualified professional.
Sources & assumptions
Overall diameter uses the metric tire designation’s section width, aspect ratio, and wheel diameter. Listed sizes are nominal; confirm measured dimensions, load rating, wheel width, offset, and clearance with the vehicle and tire manufacturers.
Frequently asked questions
How much diameter difference is acceptable?
Fitment guidance varies. Even a small change can affect clearance, gearing, speedometer readings, driver aids, and spare-tire compatibility.
Does this guarantee fitment?
No. Confirm wheel width, offset, load and speed ratings, suspension and brake clearance, and manufacturer guidance.