How to read a tire size
245/45R18, load index, speed rating, and the two places the number is printed. About a five-minute read.
Read the Tire GuideEverything on the sidewall, explained in plain language.
You do not need to become a tire expert to buy a tire. You do need to read four numbers, know what tread depth means, and know when to ask someone to look at the tire in person. That is what this guide covers.
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Read these two in full
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What a used tire actually saves you, what it costs you, and the questions to ask before you buy one.
Read the Tire GuideThe nine things worth knowing
24545R18
- 245
- Section widthThe width of the tire across the tread, in millimetres, when it is mounted and inflated to its correct pressure.
- 45
- Aspect ratioThe sidewall height as a percentage of the section width. Here the sidewall is 45% of 245 mm — about 110 mm tall. A lower number means a shorter sidewall.
- R
- ConstructionRadial. Practically every passenger car tire sold today is radial. You may occasionally see D (diagonal bias) on trailer or classic tires.
- 18
- Wheel diameterThe diameter of the wheel the tire mounts on, in inches. This one is not negotiable — an 18-inch tire only fits an 18-inch wheel.
Reading the size
245/45R18 is four facts in one string: the tread is 245 mm wide, the sidewall is 45% of that width tall, R means radial construction, and it mounts on an 18-inch wheel. Change any one of those numbers and it is a different tire.
Where the size is printed
Twice: moulded into the sidewall of the tire you already have, and printed on the placard sticker inside the driver's door frame. The placard is what the vehicle manufacturer specified. If the two disagree, someone changed the size at some point — bring us both numbers.
New or used
Neither is automatically right. A new tire gives you full tread and, where it applies, a manufacturer warranty. A used tire in good condition costs less and can be the sensible answer when you need one tire to match three you already own.
Tread depth
Measured in 32nds of an inch. New passenger tires typically start around 10/32 to 11/32. NHTSA describes 2/32 as the point at which a tire is worn out, and most tires carry moulded wear bars that become flush with the tread there.
The DOT date code
Every US-legal tire carries a Tire Identification Number starting with DOT. The last four digits are the build date: the first two are the week of the year, the last two are the year. DOT ...3421 means the 34th week of 2021.
One, a pair, or a set
Tires on the same axle should match each other in size and, ideally, in tread depth and type. Buying one tire is common and often fine; buying two is usually better placed on the same axle. All-wheel-drive vehicles are stricter — the manufacturer's manual is the authority there, not us.
Load index and speed rating
The characters after the size — 100Y, 102H, 115T — are the load index and the speed rating. They are not decoration. Fitting a tire rated below what your vehicle's placard calls for is a safety problem even when the size matches perfectly.
Pressure and TPMS
The correct cold inflation pressure is the number on your door placard, not the maximum pressure moulded on the sidewall. Vehicles sold in the US since the 2008 model year have a tire pressure monitoring system; the dashboard light means check the pressure with a gauge, not guess by looking.
What to photograph for a quote
Four photos answer almost every question we would otherwise have to ask you: the sidewall with the size, the DOT stamp, the tread straight on, and the door placard. If something looks damaged, photograph that too.
- 10-11/32Typical new passenger tire
- 6-8/32Plenty of usable life left
- 4-5/32Serviceable, but plan the replacement
- 2/32Worn out — wear bars flush
DOTU2LLLMLR3421
- 34
- The 34th week of the year
- 21
- Built in 2021
The four photos
- 1The sidewall, square on, with the size readable
- 2The DOT stamp — it may be on the inner sidewall, so check both sides
- 3The tread across its full width, taken straight down
- 4The placard sticker inside the driver's door frame
When to stop reading and have it looked at
This guide explains how tires are labelled. It cannot tell you whether your tire is safe. Age, sidewall damage, repairs, uneven wear, a puncture near the shoulder, or a size that does not match the placard all need a person to inspect the actual tire. Bring it in, or have any qualified tire professional look at it.
Where these numbers come from
General tire information on this page comes from the sources below. It is not advice about your specific tire or vehicle.
Know the size? Check what is on the racks.
Search by size to see what the shop has listed, or send the photos and we will read the sidewall for you.