Writing
Longer than an answer page, and written for someone who wants to understand the method rather than just get the number.
The calculators answer questions. These pieces explain why the answers are what they are, which is the part that stops you having to come back.
They are written in the order the questions get asked. The first is about the sidewall itself — every number moulded into it, what each one controls, and what changes downstream when you change it. After that the pieces get narrower: what the fitment question really means, what speedometer error costs over a year of driving, when gearing genuinely enters the conversation, and what "going bigger" adds up to once every consequence is counted.
Every number here is checked
Every numeric example in these pieces is computed by the same library that runs the calculators and pasted in, never typed from memory. Where a piece works through an example, it states the inputs explicitly so you can reproduce it in the tool and check it.
That is not a courtesy. A site of this kind can very easily state a figure its own calculator disagrees with — a transposed digit in prose reads exactly like a correct one, and no ordinary review catches it. So every worked example is declared alongside its inputs and re-derived from the library whenever the site is built. If a number in an article and a number in a tool ever disagree, the build stops.
What is not here
Product recommendations, brand comparisons and buying guides. Those need a tester, a test track and a disclosure policy this site does not have, and the internet is not short of them.
What is here is the arithmetic, worked slowly, with the places it stops applying marked clearly.
Reading them in order
The sidewall piece is the foundation and everything else assumes it. Read that one first if you are starting from nothing; the rest stand alone and can be read in any order, or ignored entirely in favour of the calculator that answers the same question in ten seconds.
- Every number on your sidewall, and what changing it actually doesEvery number on your sidewall, and what changing it doesA tire's sidewall carries dimensions, ratings, an identity and a set of legal notices in one typeface. Here is which is which, which ones you can change, and what moves when you do.
- Will 33s fit? What the question really meansWill 33s fit? What the question really means"Will 33s fit" is four separate questions wearing one coat. Here is what each one is, which have arithmetic answers, and which do not.
- Speedometer error: the arithmeticSpeedometer error: the arithmeticFit a different tire and the needle lies by a fixed percentage, not a fixed number of mph. Three things people get wrong about that, and the case where the odometer costs you warranty.
- When do you need to re-gear?When do you need to re-gear?"Need" is not a property of the tire. It is a property of where the engine lands at the speed you actually drive. Here is the arithmetic that answers it, and the four questions the arithmetic cannot.
- Metric vs imperial tire sizesMetric vs imperial tire sizesThese are not two units for the same measurement. They are two different measurements, taken by two industries for two purposes — and the conversion between them loses information in one direction only.
- Load index, and why it matters more than widthLoad index, and why it matters more than widthThe size code gets all the attention. The two or three characters after it carry the only hard ceiling printed on the tire — and going wider does not raise it by a single pound.
- How tire size changes your effective final driveHow tire size changes your effective final driveYour axle ratio and your tire diameter are not two separate specifications. They are one quantity, and quoting either without the other says almost nothing.
- Revolutions per mile, and why your odometer driftsRevolutions per mile, and why your odometer driftsThis is the one tire figure where the calculated number and the manufacturer's number legitimately disagree. Understanding why explains what a tire calculator can and cannot promise you.
- Plus-sizing wheels without changing rolling diameterPlus-sizing wheels without changing rolling diameterOne equation, two unknowns, and both of them quantised. That is why plus-sizing never lands exactly — and why the old rule of thumb gets you within a fifth of an inch anyway.
- Reading a DOT date codeReading a DOT date codeEvery other marking on a tire describes the model. This one describes the object in front of you — the week it was cured, the plant that cured it, and nothing about how good it is.
- Diameter vs width: which changes whatDiameter vs width: which changes whatEvery consequence of a tire change belongs to exactly one of two dimensions. Sorting them takes five minutes and stops most of the arguments — including the one about why you cannot change width on its own.
- The cost of going biggerThe cost of going biggerThe invoice for a larger tire is the part people budget for. Everything structural about why it costs more, and the recurring costs that never appear on any invoice at all.