About RollingSpec

RollingSpec started from a familiar problem: comparing two tire sizes, or checking what a different wheel offset would actually do to fitment, usually means digging through forum threads full of conflicting rules of thumb, or plugging numbers into a spreadsheet by hand. This site is a set of small, focused calculators that do that arithmetic directly, show their work, and stop short of guessing at things they can't actually know.

How the calculators work

Every tool on this site runs entirely in your browser. There's no server-side processing of the numbers you enter, no account, and nothing about your inputs is stored or transmitted anywhere — the JavaScript that computes diameter, circumference, backspacing, and the rest ships to you and runs locally, the same as a calculator app would. The underlying formulas are standard tire and wheel geometry: aspect ratio and section width give sidewall height, sidewall height plus wheel diameter gives overall diameter, and so on. The glossary spells out each term used across the calculators, and each calculator page explains the specific formula it uses under the tool itself.

Why we're upfront about accuracy

Tire size math is nominal by design — a "225/65R17" describes a published spec, not the exact millimeter dimensions of any one manufacturer's tire, which vary slightly in practice. The calculators are built and tested against those published conventions so the arithmetic itself is correct, but arithmetic is only part of the fitment question. It can't account for your specific vehicle's suspension travel, brake caliper clearance, wheel-well shape, or load requirements — that's why every result here is labeled as an estimate, and every tool page says plainly what it does and doesn't check.

What this site is not

RollingSpec is not a substitute for your vehicle owner's manual, a manufacturer's fitment guide, or a qualified tire or wheel professional. If a calculation here suggests a combination might work, treat that as a starting point for a real fitment check — not the final word.

Questions, corrections, or a formula that looks off? See the contact page.