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Tire Size Change Calculator
Compare old and new tires for effective gearing change
A tire size change shows how a taller or shorter tire changes effective gearing and speedometer accuracy.
Formula: Effective gear = gear × (old diameter ÷ new diameter) · Speedometer error = (new ÷ old − 1) × 100%
What this number means
This compares two tire sizes and shows the change in diameter, circumference and effective gear ratio between them. A taller tire acts exactly like a numerically lower gear, whether or not you touched the rear end.
What to change next
If the new tire changes effective gearing by more than a couple of percent, decide deliberately whether to accept it or correct it with a ratio change. Also expect speedometer error and a change in trap RPM, and verify both on the first slips.
Worked example
Swapping a 28 in tire for a 30 in tire makes a 4.10 gear behave like ≈ 3.83 — about 7% taller.
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 a tire size change on RegearReady.