What does the arrow on my tire mean?

Directional tread. The tire must be fitted so it rotates the way the arrow points, which means a left and a right side and no swapping across the car.

What is the difference between 235/45 R18 and 235/45 ZR18?

ZR is an older way of marking a high speed rating. On a modern tire the speed rating letter after the load index is the one that matters.

Is the tread wear indicator the same as tread depth?

No. The indicators are small raised bars in the grooves that become flush with the tread at the legal minimum, which is 2/32 of an inch, which most states enforce with a penny test. They tell you that you have reached it, not how far away you are.

What every marking on your tire sidewall means

A tire sidewall reads like 235/45 R18 98Y XL. That is a tire 235mm wide, with a sidewall 45 per cent of that width tall, of radial construction, on an 18 inch rim, rated to carry 750 kilos per tire at up to 186mph, in Extra Load construction. Everything else on the wall is a date code, a maker's mark or a technology name.

The size, part by part

235 is the section width in millimetres, measured across the widest part of the inflated tire, not across the tread.

45 is the aspect ratio. It is a percentage of the width, not a measurement, so the sidewall here is about 106mm tall. This is why two tires with the same first number can be very different heights.

R means radial. You will not meet anything else on a modern car.

18 is the rim diameter in inches. It is the one imperial number in a metric string and it has survived every attempt to change it.

The load index and speed rating

98 is a load index, not a weight. It is a lookup: 91 is 615 kilos, 95 is 690, 98 is 750, 101 is 825. Each tire carries that much, so four of them carry four times it.

Y is the speed rating. H is 130mph, V is 149, W is 168, Y is 186. It is not a suggestion about how fast to drive; it is a statement about the heat the casing can shed before it fails.

Neither may be lower than the figure on your placard.

The letters that come before and after

XL or Extra Load, and HL or High Load, describe the construction. HL appears before the size and XL after the speed rating, which is why HL is the one people miss.

A maker's mark identifies a tire developed for one manufacturer: T0, T1 and T2 for Tesla, MO and MO-E for Mercedes, a star for BMW, AO for Audi, TPC for General Motors, a plus for Volvo, N followed by a digit for Porsche. These are not decoration and the guide on OE markings explains why.

The DOT code, and how old your tire is

Somewhere on the wall is DOT followed by a string ending in four digits. Those four are the week and the year the tire was made. 2225 is the twenty-second week of 2025.

Rubber ages whether or not it is used. A tire with plenty of tread and a date code from eight years ago is a tire near the end of its life, and on a car that sits still a lot, which many second cars and caravans do, age is what finishes them rather than wear.

Common questions

What does the arrow on my tire mean?: Directional tread. The tire must be fitted so it rotates the way the arrow points, which means a left and a right side and no swapping across the car.
What is the difference between 235/45 R18 and 235/45 ZR18?: ZR is an older way of marking a high speed rating. On a modern tire the speed rating letter after the load index is the one that matters.
Is the tread wear indicator the same as tread depth?: No. The indicators are small raised bars in the grooves that become flush with the tread at the legal minimum, which is 2/32 of an inch, which most states enforce with a penny test. They tell you that you have reached it, not how far away you are.

Last checked 2026-09-22. Where this page and your own vehicle disagree, the vehicle wins.