How to Read Tire Size: Step-by-Step Guide

Every tire sold in the United States has its full size stamped right into the sidewall, yet most drivers have never been shown how to read it. Once you can, buying the correct replacement takes about thirty seconds and you never have to guess or rely on someone else to look it up. Here is the entire code, decoded with a real example.

Where to find your tire size

There are two reliable places. The first is the tire sidewall itself, where a string like 225/65R17 102H is molded into the rubber. The second is the vehicle placard, a sticker on the driver-side door jamb that lists the size your vehicle was built for. If the two ever disagree, the placard is the size to trust, because a previous owner may have fitted something different.

What each part of 225/65R17 102H means

Read the code from left to right. Each block describes one physical property of the tire.

Some sizes carry a letter before the width, such as P225/65R17 or LT225/65R17. P means P-metric, the passenger-car standard in the US. LT means light truck, built for heavier loads and higher pressures. No letter usually means Euro-metric, which is effectively interchangeable with P-metric in most fitments. If your placard calls for LT, do not substitute a P tire.

Why the aspect ratio matters more than it looks

The middle number quietly changes how your car rides and handles. A higher number like 70 means a taller sidewall, which soaks up bumps and helps protect the wheel from potholes. A lower number like 45 means a short, stiff sidewall that sharpens steering response but passes more of the road through to you. Move it too far from the original and your speedometer will read wrong and the ride will change character, so keep the aspect ratio close to your placard unless you are deliberately building a plus-size setup with a tire professional.

How to read your tire size in five steps

1. Face the sidewall and find the block of numbers formatted like 000/00R00.
2. Read the first three digits as the width in millimeters.
3. Read the two digits after the slash as the aspect ratio.
4. Note the letter, usually R, and the wheel diameter that follows it.
5. Read the two or three digits and the final letter as the load index and speed rating.

Do this once on your own car and the pattern sticks. If you want to shop straight from your size, enter it below and see what fits.

What do the numbers on a tire mean beyond the size?

The size is the headline, but the sidewall carries more. Find the DOT code, which ends in four digits giving the week and year the tire was made, so 2419 means the 24th week of 2019. You will also see the UTQG rating, three marks for treadwear, traction, and temperature that let you compare tires of the same type. The M+S letters or the three-peak mountain snowflake symbol tell you the tire's winter capability. You will also spot a line like MAX LOAD and MAX PRESSURE molded into the rubber. That figure is the tire's own ceiling, not the pressure you should run. Your correct inflation always comes from the door placard, not the sidewall.

Load index and speed rating, and why you should never downgrade them

These last two marks are limits, not suggestions. The load index sets the most weight one tire can safely carry, and the speed rating sets the top speed it is built to sustain. Your vehicle was engineered around specific values, so fitting a tire below either one is a real safety compromise, even when it saves a few dollars. Going higher is fine, going lower is not. Common speed ratings line up like this:

If your original tires are rated H, buy H or better. The same rule applies to the load index: match the number on your placard or exceed it, and if you regularly tow or haul, ask a tire professional whether a reinforced or higher-load option is the safer pick.

Which number is the actual tire size?

People often ask which single number to give a tire shop. The answer is the whole string, not one number. A bare 17 only tells the wheel size, and 225/65R17 without the load index and speed rating leaves out how much the tire can carry and how fast it is rated to run. Always quote the full code, and add the load index and speed rating when you want an exact match to your original tires.

Where else your tire size appears

If the sidewall is dirty or worn, the same size is printed in three other spots: the driver-side door placard, the owner's manual, and often inside the fuel-filler flap or the glovebox lid. These list the vehicle's original specification, which is the safest size to buy unless a tire professional has advised a deliberate change.

The verdict

Reading a tire size is a two-minute skill that saves you money and keeps you safe. Learn your own code, write it down, and confirm it against the door placard before you buy. Match the width, aspect ratio, wheel diameter, load index, and speed rating, and any tire that carries your size will physically fit. When you are ready, look up your exact size and compare what is in stock.