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 = wheel diameter + 2 × (section width × aspect ratio ÷ 25.4). 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.
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.