How to read a tire size
Every tire on the road carries its own spec sheet moulded into the sidewall. Once you can read 245/45R18 you can read all of them.
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The four parts of 245/45R18
A modern passenger tire size is a single string with four pieces of information packed into it. Read left to right.
- 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.
Two of those units are metric and one is imperial, which is exactly as confusing as it sounds. It is a historical accident, not a mistake on your tire.
Finding it on the sidewall
The size is moulded into the rubber, usually in the largest lettering on the sidewall, often between the brand name and the rim. It can be hard to read on a dirty or dark tire — wipe it and use your phone light at an angle so the raised letters cast a shadow.
The same sidewall also carries the load index and speed rating, the DOT Tire Identification Number, the maximum inflation pressure, and treadwear/traction/temperature grades. The size and the DOT code are the two you will be asked for most.
Finding it on the door placard
Open the driver's door and look at the frame, near the latch. Almost every vehicle sold in the US carries a placard there listing the tire size the vehicle was built for and, importantly, the recommended cold inflation pressure for the front and rear tires.
Some vehicles put the placard in the glove box, the fuel filler door, or on the door itself. The owner's manual says where.
The placard is the vehicle manufacturer's specification. The sidewall only tells you what is on the car right now — which may or may not be what should be.
Load index and speed rating
After the size you will usually see something like 100Y or 102H. The number is the load index — a code for the maximum weight that tire can carry. The letter is the speed rating.
Neither is optional. A tire whose load index is below the vehicle's requirement can be overloaded at perfectly normal highway weights, and speed rating affects heat handling, not just top speed. If you are replacing one tire, match the existing rating or ask before you go lower.
| Load index | Maximum load |
|---|---|
| 91 | 1,356 lb (615 kg) per tire |
| 95 | 1,521 lb (690 kg) per tire |
| 100 | 1,764 lb (800 kg) per tire |
| 102 | 1,874 lb (850 kg) per tire |
| 115 | 2,679 lb (1,215 kg) per tire |
| Speed rating | Maximum speed |
|---|---|
| T | 118 mph / 190 km/h |
| H | 130 mph / 210 km/h |
| V | 149 mph / 240 km/h |
| W | 168 mph / 270 km/h |
| Y | 186 mph / 300 km/h |
Load index and speed rating tables are standardised across manufacturers. Use your vehicle's placard as the requirement, not this excerpt.
P, LT and other prefixes
A size may start with a letter. P means P-metric, a passenger-car tire. LT means light truck — built for higher loads and inflated differently. No letter at all usually means Euro-metric, which is close to P-metric but not identical in load calculation.
LT265/70R17 and 265/70R17 are not interchangeable just because the numbers match. If your placard calls for LT, stay with LT.
When the tire and the placard disagree
It happens often, especially on used vehicles. Someone fitted a different size at some point — sometimes deliberately, sometimes not. A different size changes the rolling diameter, which affects your speedometer, your traction control, and on some vehicles the drivetrain.
There is no safe way to settle that from a web page. Bring both numbers — what is on the car and what is on the placard — and have the fitment confirmed in person before anything is installed.
What to photograph
If you would rather not decode it yourself, photograph it and send it with a quote request. Four shots cover it:
- 1The sidewall square on, size readable
- 2The DOT stamp — check both sides of the tire, it is often only on the inner one
- 3The tread, taken straight down, across the full width
- 4The placard 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.
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