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Tire Diameter & Rollout Calculator
Calculate overall tire diameter from the sidewall size
Tire diameter converts a metric tire size to its true diameter and rollout — the inputs every gear and speed calculation stands on.
Formula: Metric diameter = rim + 2 × (width × aspect% ÷ 25.4) · Rollout = π × diameter
What this number means
This converts a metric tire size — width, aspect ratio and wheel diameter — into overall diameter, circumference and revolutions per mile. Marked sizes vary a little by manufacturer, so treat it as accurate to the size system, then measure if you need better.
What to change next
Use the diameter in every gearing, slip and speedometer calculation instead of the number painted on a slick or guessed from the sidewall badge. When changing tire sizes, rerun those calculations before the first pass.
Worked example
A 275/60R15 tire calculates to ≈ 28.0 inches in diameter and ≈ 88 inches in circumference.
Related calculators
Read the guide: Gear Selection Guide →
Building a Jeep or 4x4 too? The same gear math drives your crawl ratio. RegearReady — our sister site — turns it into regear decisions: Check tire diameter on RegearReady.