About this site

This site exists because the same handful of questions get asked constantly, all of them have a single correct numerical answer, and almost every page that answers them is either a store trying to sell you something or a forum thread from 2011.

How it is built

There is one calculation library. It parses a size code in either notation, works out the geometry, and computes revolutions per mile, speedometer error, effective final drive and engine speed from that. Every page and every calculator on the site calls it. Nothing on a reference page is typed in by hand, which means a size page and the comparison tool cannot print different numbers for the same tire — they are the same function.

Everything is computed when the site is built or in your own browser. No page asks a server for a figure while you are reading it, which is why the pages load quickly and why the site keeps working when a third party has a bad day.

What it will and will not say

There is a hard line here between arithmetic and advice, and the site stays on one side of it. A tire's diameter is a fact about the tire. Whether it rubs the inner fender at full lock is a fact about your truck, your wheels, your suspension and how much of it somebody has already changed — and no pair of size codes contains that fact. So the calculators stop where the arithmetic stops. What this site does and does not tell you sets out the full position and what to read for the questions on the other side of the line.

Where a figure could not be confirmed against a source, the field is left out rather than filled with something plausible. That is why some platform pages carry axle ratios and others do not.

Spelling

The site uses the American spelling, tire, throughout — including in its data and its addresses — because that is how the great majority of people searching for this arithmetic spell it. The British spelling is tyre, and it means exactly the same thing; the sidewall marking reference notes the difference where it matters.

Other projects

This is one of a small family of independent calculation sites built the same way: one tested library, every page reading from it, nothing fetched at request time. The nearest siblings by approach are GetTheAmount, which does the same for home-improvement quantities, and MathQuarry, which shows the method alongside the answer for everyday mathematics. They share no content with this site and there is no reason to visit them unless the subject is what you need.

Corrections

If a number here is wrong, that is worth more to fix than anything else on the site. The contact page has an email address that reaches a person.