Guides
The questions a per-size page cannot answer. Free, like everything else in the reference.
How to read a tire sidewall
A sidewall carries four unrelated marking systems. Read in the right order they answer four separate questions: what size is it, what can it carry, how fast is it rated, and how old is it.
3 minute read · 5 sections
Plus sizing and minus sizing, without changing the rolling diameter
A bigger wheel with a shorter sidewall, or a smaller wheel with a taller one, holding the overall diameter roughly constant. What the arithmetic can tell you, and the four things it cannot.
3 minute read · 4 sections
Tire size and speedometer error
A taller tire makes the speedometer read low, in exact proportion to the diameter change. The arithmetic is simple; the assumption behind it is the part worth understanding.
2 minute read · 3 sections
Load index, load range and ply rating: three different things
The load index is a code for a mass. The load range letter is a construction grade. The ply rating is an equivalence to a construction nobody uses any more. They appear together and answer different questions.
2 minute read · 4 sections
P-metric against LT: why the same numbers are not the same tire
P265/70R17 and LT265/70R17 describe the same nominal shape and are rated by different standards, at different pressures, with a derating that applies to one and not the other.
2 minute read · 3 sections
How old is my tire, and how would you know
The DOT date code is the only marking that dates a tire. Here is how to find it, how to read both formats, and what the date does and does not tell you.
2 minute read · 3 sections