Tire and gearing calculators

Each of these takes what you already know — the size on the vehicle, the size you are considering, an axle ratio — and returns the arithmetic. None of them asks for an email before showing a result, and none of them calls a server: everything is computed in your browser from data that ships with the page.

Every result is addressable. Change an input and the address bar changes with it, so the link you copy reproduces exactly what you were looking at. That is deliberate — a number nobody can send to someone else is a number that has to be recalculated every time it comes up in a conversation.

Which one you want

Comparing two sizes is the one most people arrive for, and it is the one with the drawing. It answers "how much bigger, in every dimension" in a single view.

Speedometer error takes the same two sizes and expands the single percentage into a table across a range of speeds, which is the form you want if you are trying to work out what an indicated 70 actually means on a long drive.

Gear ratio goes further downstream: what the engine is doing at a given road speed in each gear, before and after a tire change, and which off-the-shelf ratio would restore the original figure.

The size converter answers the metric-versus-imperial question, and — the part that makes it useful — names the sizes that genuinely exist near the answer instead of stopping at a decimal.

The load and speed index decoder reads the part of the sidewall after the size, which almost nobody can read and which constrains more than anything else printed there.

What they share

All five sit on one calculation library. The diameter the comparison tool shows and the diameter the gearing calculator uses are the same function call, so no two tools on this site can disagree about the same tire — and neither can a tool and a reference page.

All five open with a worked result rather than an empty form. A blank calculator asks you to supply the example before it will show you anything, which is backwards: the fastest way to understand what a tool does is to see it doing it. The defaults are a real, common pairing, and the first thing you see is a real answer.

All five put the result where you can take it away: a control that copies the number as a sentence, and a control that copies the address of exactly what you are looking at.

What none of them do

None of them decides anything for you. There is no "recommended" ratio, no verdict on a fitment, no green tick. The gearing calculator will tell you the ratio that restores your original engine speed and name the nearest one sold; it will not tell you to buy it.

That restraint is the reason the numbers can be stated flatly. An answer that has been adjusted to be helpful is no longer an answer.