Every number on your tire sidewall, and which ones actually matter
A plain walkthrough of the markings molded into every tire — size, load, speed, date, wear bars — and what each one should change about your decision.
A tire sidewall carries more printed information than any other single part of a car, and almost none of it is explained anywhere the owner is likely to look. Much of it is molded there because a regulator asked for it. A few of the marks decide whether the tire is safe and legal on your car at all.
You only really need to read four things: the size code, the load and speed ratings, the date, and the wear bars. Here they are in the order you would meet them if you crouched down beside a front wheel with a flashlight.
The size code
The main string looks something like 225/45R17 94W. It is a mix of metric and imperial units, which is why it never quite makes sense at first glance.
| 225 | Section width in millimeters, measured sidewall to sidewall on a mounted tire |
|---|---|
| 45 | Aspect ratio — sidewall height as a percentage of the width, so roughly 101 mm here |
| R | Radial construction, which is what nearly every passenger car on the road uses |
| 17 | Wheel diameter, in inches, the one imperial number in the middle of a metric code |
| 94 | Load index — a lookup value, not a weight in itself |
| W | Speed rating, also a lookup |
The aspect ratio is the number people change without meaning to. Drop it and the sidewall gets shorter: steering feels sharper, the ride gets busier over broken pavement, and there is less rubber between a pothole edge and your wheel rim. Change width or profile enough and the rolling circumference shifts too, which quietly changes what your speedometer reports. The size on the placard inside the driver's door is the size the car was engineered around, and deviating from it is a conversation for a tire specialist rather than a forum thread.
Load index and speed rating
Both are codes that point into published tables rather than values you can read directly. Load index 94, for instance, corresponds to 670 kg per tire. Speed ratings run through the alphabet in an order that is not alphabetical: T is 190 km/h, H is 210, V is 240, W is 270, Y is 300. Any tire shop has the full table and will show you.
Load index is the one to be strict about. It reflects how the casing is built and what it can carry at pressure without overheating. Never fit a tire with a lower load index than the vehicle placard specifies. This bites most often on vans, seven-seaters and anything towing, where a passenger-car tire in the right size can still be the wrong tire.
Speed rating matters even to people who never approach those numbers, because it tracks how the tire is built and how much heat it tolerates on a long, loaded, hot highway run. In some countries fitting below the manufacturer's specified rating affects roadworthiness and insurance. Winter tires are a defined exception in several places, sometimes with a sticker on the dash stating the lower limit. Local rules differ enough that you have to check your own.
The date code
Somewhere on the sidewall is a code beginning with DOT. The last four digits are the ones to read: the first two are the week of manufacture, the last two the year. So 3618 means the thirty-sixth week of 2018. Only one side of the tire carries the full code, so if you cannot find it, look at the inboard face.
Rubber ages whether or not it is turning. Compounds harden, and fine cracks can appear in the sidewall and between the tread blocks — worth looking for on any car that sits a lot. A tire sold as new can legitimately have spent a couple of years in a warehouse, so it is fair to ask the date before the wheels come off. Age guidance varies between car makers and tire makers, so the number to follow is the one printed in your own owner's manual.
Wear bars and tread depth
Look into the main circumferential grooves and you will see short raised ridges bridging them. Those are the wear indicators, molded at the legal minimum depth used in most markets — 1.6 mm, or about 2/32 of an inch. When the tread surface is flush with them, the tire is finished as a road tire. Wet grip starts falling away well before that point, because a shallow groove has nowhere to put the water.
A tread depth gauge costs about the same as a sandwich. Measure inner, center and outer, then repeat at a few points around the circumference, because wear is almost never even. In the United States a penny does as a rough check: push it into the groove with Lincoln head down, and if the top of his head is visible you are at or below 2/32 of an inch.
The pattern of the wear is a diagnosis in itself. Both shoulders worn with the center fine usually means the tire has been run soft for a long time; a worn center strip points the other way. Wear concentrated on one edge, or a feathered, saw-toothed feel when you run a hand across the blocks, usually means alignment or a worn suspension part — and that belongs on an alignment rack with someone who can measure it.
The winter marks
M+S, sometimes shown as M/S or M&S, is a manufacturer's own declaration about mud and snow. Nobody tests it. The three-peak mountain snowflake symbol is different: a tire only carries it after passing a defined snow traction test. Where winter tires are legally required, the regulation names one symbol or the other, and the two are not interchangeable. Check what your region asks for before the first cold snap rather than during it.
The marks that catch out fitters
Some tires are directional and carry a rotation arrow; others are asymmetric and say OUTSIDE and INSIDE. A tire mounted the wrong way round still holds air and looks normal from the driver's seat, so walk around the car after new tires go on. While you are down there, ignore the maximum pressure printed on the sidewall — that is a ceiling for the tire, not a recommendation for your car. The pressure to use is on the door jamb placard, sometimes inside the fuel filler flap, and in the manual.
Where the do-it-yourself line sits
Checking pressures, reading codes and measuring tread are all yours. Mounting and balancing are not: seating a bead involves enough stored energy to injure someone badly, and it needs the machine and the cage. Repairs have a hard boundary too. Damage in the tread area can sometimes be repaired properly, from the inside, with the tire off the rim and the interior inspected. Sidewall damage cannot: a bulge, a cut deep enough to show cord, or a hard curb strike means the tire comes off the car and stays off. If you find a bulge, stop driving on it and have it replaced.
None of this takes long once you know where to look. Next time you are waiting for someone in a parking lot, go and date-code your spare.