How to read a tire size

Every tire has its size molded into the sidewall in a string that looks like a part number. Something like 225/65R17 102H, a size you'll find on plenty of compact SUVs. Once you know what each piece means, you can check whether the tires on your car are right, compare quotes on equal terms, and avoid buying something that fits the wheel but not the vehicle.

Start with the door, not the tire

Open the driver's door and look at the edge of the door or the post it closes against. There's a tire information placard there. On some vehicles it's on the glove box door or inside the fuel door instead. It lists the size the vehicle maker fitted at the factory, the spare size, and the cold inflation pressure for the front and rear tires.

That placard is the reference. The tires on your car right now might not match it. A previous owner may have put on a different size, or a shop may have installed whatever was in stock. If the sidewall and the placard disagree, go by the placard until someone who knows fitment says otherwise. The owner's manual usually repeats the same information and may list approved optional sizes.

The size, one piece at a time

Marking	What it means
P	P-metric passenger tire. LT means light truck. No letter usually means a metric passenger size.
225	Section width in millimeters, sidewall to sidewall. About 8.9 inches.
65	Aspect ratio. The sidewall is 65 percent as tall as the tire is wide.
R	Radial construction.
17	Fits a 17-inch wheel.
102	Load index. 102 means 1,874 pounds per tire.
H	Speed rating. H is 130 mph.

The letter in front

P means the tire was built to the P-metric passenger standard. LT means light truck: a tire made to carry heavier loads at higher pressures, common on heavier pickups, vans and trucks that tow. Plenty of pickups and SUVs leave the factory on P-metric or metric passenger tires, so don't assume a truck needs LT. The placard says which one yours takes.

One rule doesn't bend. Don't mix passenger tires and LT tires on the same vehicle. They're built differently and carry load differently.

Width and aspect ratio

The 225 is the width in millimeters. The 65 is a percentage, not a measurement. On a 225/65, the sidewall is 65 percent of 225, or roughly 146 millimeters tall. A little under 6 inches. Aspect ratios typically run from 30 to 80. A lower number means a shorter sidewall, which usually goes with a bigger wheel, quicker steering and a firmer ride. A higher number means more rubber between your wheel and the pothole.

R and the wheel size

The R means radial, and nearly every car and light truck tire sold today is radial. The 17 is the diameter of the wheel the tire mounts on, in inches. It isn't the height of the tire. A 225/65R17 and a 265/70R17 both fit a 17-inch wheel, and they're very different tires.

Load index and speed rating

The characters after the size are called the service description. On our example it's 102H.

Load index

The load index is a code for how much weight one tire can carry at its maximum inflation. Load index 102 works out to 1,874 pounds per tire, so a set of four is rated for 7,496 pounds. Most passenger tires fall somewhere between 75 and 100. Higher numbers carry more.

Replacement tires shouldn't have less load capacity than the originals at the pressure on your placard. A lower load index on a loaded-up SUV or minivan means tires working harder and building more heat than they were designed for. Heat is what breaks tires down.

Speed rating

The letter is the maximum speed the tire is rated for in standardized lab testing. The common ones:

Rating	Max speed
S	112 mph
T	118 mph
H	130 mph
V	149 mph
W	168 mph
Y	186 mph

If your placard or owner's manual calls for a speed-rated tire, replacements should match it or go higher. Go lower and the vehicle's speed has to be limited to the new tire's rating, and handling can change. Dedicated winter tires are the usual exception, since a lower speed rating is generally accepted on them. Nobody is suggesting you drive 130 on the 10. The rating matters because the vehicle maker built the car around a certain kind of tire.

Light truck sizes read a little differently

An LT size looks like this: LT245/75R16 120/116Q Load Range E. The two load numbers are for single and dual fitments, since some trucks run two tires side by side on the rear axle. The load range letter tells you the tire's load and inflation limits. If you tow or haul, the load range matters as much as the size.

The DOT code tells you the tire's age

Look for the letters DOT followed by a string of letters and numbers. That's the tire identification number. On any tire made since 2000, the last four digits are the week and year it was built. A code ending in 3520 means the 35th week of 2020.

The full code isn't always on both sides. If you only see part of it, check the other sidewall, which may be the one facing the car. That number matters for recalls too. Tire makers use it to identify affected tires, and you need it to register a new set.

Age matters as much as tread. For how old is too old, read when to replace your tires.

Other marks on the sidewall

• Max pressure. The pressure molded on the sidewall is the most the tire is allowed to hold. It isn't the pressure for your car. Use the placard.
• M+S. Mud and snow. Common on all-season tires, and it means limited mud and snow capability.
• Mountain and snowflake symbol. A tire that meets the industry definition for use in severe snow. That's a real winter tire.
• Treadwear, traction, temperature. The UTQG grades. A treadwear grade of 600 wore twice as well on the government test course as a 300. Traction runs AA, A, B, C, based on straight-line braking on wet pavement. Temperature runs A, B, C.

When your tires don't match the door

Bigger wheels, lifted trucks and plus-sized fitments are all possible. Picking a size other than the factory one is real fitment work, though. Overall diameter, load capacity, clearance at full steering lock and speedometer reading all change. If you're looking at new wheels, start at our wheels page and we'll talk through what fits.

Ready to shop? Grab the size off your door placard and search it on our tire page, or call (909) 321-2191 with your year, make and model. Not sure what's on the car now or how old it is? Book a tire inspection at our Montclair shop and we'll read every sidewall, check the dates and tell you where you stand. Bought tires online? Book mount and balance and bring them in.