Who computes these numbers, and how

Every figure on this site is computed from the tire's own sidewall code by one formula. No chart is copied from anywhere, no dataset is licensed, and nothing here can go stale because there is nothing to refresh.

The site exists because the answer to "what is 285/70R17 in inches" is arithmetic, and arithmetic that is shown is arithmetic you can check. The converter works the formula with whatever size you type, so a figure you disagree with can be argued with rather than trusted.

Every figure on this site is computed from the tire's own sidewall code with one formula. No chart is copied and no manufacturer data is used. Brand and vehicle names, where they appear, only identify what a size is used on.

Overall diameter is the wheel plus two sidewalls. In inches that is wheel + (width × aspect × 2) ÷ 2540. The constant is 2540 exactly — 100 × 25.4 — and charts that round it to 2550 come out short by a margin that widens as diameters grow.

The assumptions behind every figure

Sizes are nominal. A molded tire varies from its stated size by manufacturer.

Unloaded diameter. Under load the rolling radius is smaller.

Rim width, load index and speed rating are not in the sidewall code and are not calculated here.

This is a dimensional calculation, not a fitment guarantee.

A number that looks wrong is worth an email, and it gets a reply. hello@tiresizeinches.com is read by the person who wrote the formula.