How to Check Tire Size and Understand the Numbers

The vehicle specification comes first

If you're about to copy a number off a tire for an order, we'd check the vehicle's placard first because a previous owner may have changed the fitted size. That gives you the manufacturer's specification to compare with the rubber on the car, instead of carrying a previous size change into your next purchase.

Where to look and what to write down

Open the driver's door and look for the tire information placard on the door or door jamb. If you can't find it there, use your vehicle's owner's manual for the recommended size and specifications. Michelin's tire-size guidance (opens in a new tab) identifies these as the vehicle-specific references.

Keep any front and rear entries separate when you write them down. Some vehicles intentionally use wider rear tires, a setup called staggered fitment, and their placard lists both sizes. A difference between the front and rear is something to compare against those entries.

Next, find the size characters molded into each current tire's sidewall. Write the code beside its position on the vehicle so you can compare a front tire with the front specification and a rear tire with the rear specification. You're collecting two distinct pieces of information: what the vehicle calls for and what each fitted tire identifies itself as.

What 225/65R17 means

Use 225/65R17 102H as a worked metric example. Read the 225/65R17 portion in groups, because its numbers use different units and its letter describes construction. The trailing 102H carries the tire's load and speed information.

The first number, 225, is the section width in millimeters at the tire's widest point. The 65 after the slash is the aspect ratio: the sidewall height is 65% of that width. It isn't a height of 65 millimeters, so keep the width and aspect ratio together when comparing codes.

The R identifies radial construction. The 17 after it is the rim diameter in inches, and that diameter must match the wheel exactly. In an example ending R18, the corresponding rim diameter is 18 inches; the same exact-match requirement applies.

Keep all these characters in your notes. Width and aspect ratio help determine fit on the wheel and clearance within the wheel well, while the rim number identifies the required wheel diameter. Michelin's sidewall explanation (opens in a new tab) breaks down the metric code by these individual characteristics.

The load index belongs in your notes too

Continue past the rim diameter when you copy the code. In 225/65R17 102H, the 102 is the load index and H is the speed rating. Together, that number and letter form the service description, and tires with identical size dimensions can have different service descriptions.

A load index is a lookup code, not a weight written directly in pounds or kilograms. It corresponds to the maximum weight one tire can support when properly inflated. To interpret the capacity behind your recorded number, use a standardized load index chart, as explained in Michelin's load-rating guidance (opens in a new tab).

Verify the manufacturer's required ratings on the placard or in the owner's manual as part of your comparison. The load rating must meet or exceed the vehicle's specified value. Never accept a lower load index than the factory specification: inadequate capacity can lead to sidewall failure, blowouts and loss of vehicle control.

The weight being supported includes the vehicle's passengers and cargo, plus the battery in an EV. Each tire carries its share of that total, which is why the service description matters even when the dimensions look right.

Why the speed letter matters

Read H as a speed-rating symbol. It identifies a maximum sustained speed of 130 mph under the specified load conditions, with proper inflation and loading. The letter describes an operating limit, so retain it exactly when recording the sidewall.

For your comparison, use the manufacturer's specified speed rating as the baseline to meet or exceed. A lower rating needs attention because the tire isn't designed to manage the same heat buildup during sustained high-speed operation. Exceeding either the load or speed limit can create excessive heat, reduce durability and increase the risk of tire damage or failure.

Those letters matter at normal highway speeds, too. Speed ratings reflect structural integrity and heat resistance, and a properly rated tire is engineered to the vehicle manufacturer's tolerances. Michelin's speed-rating explanation (opens in a new tab) also warns that, in some cases, fitting a lower-rated tire can affect warranty and insurance coverage.

Keep P and LT with the size

Look immediately before the size numbers for a P or LT prefix. P, as in P225/65R17, identifies a passenger tire intended for cars, minivans, SUVs and light-duty trucks used mainly to carry people. Include that prefix in your record instead of starting at the first number.

LT means Light Truck. These tires have a stiffer carcass and higher load capacity for towing, heavy payloads and applications needing more structural strength. That distinction helps explain why matching the width, aspect ratio and rim diameter is only part of checking the specification.

If your truck or SUV came with LT-rated tires, a P-metric tire of the same size leaves you short on load capacity. Keep the factory LT requirement visible in your notes so it carries through to the compatibility check. Michelin's P and LT explanation (opens in a new tab) identifies this as a safety issue.

XL and HL describe extra load capacity

Also record XL, HL or Reinforced if you see them. These markings identify tires designed to carry higher loads than standard tires of the same size. Reinforced or special-load tires require an equivalent specification, so these words and letters belong alongside the dimensions and service description.

XL stands for Extra Load. Its reinforced internal structure allows more weight at higher inflation pressures than a standard-load tire of the same size. If your vehicle's factory tires are XL-rated, replacements must retain XL capacity because the same-size standard-load version lacks the capacity needed.

HL means High Load, a designation developed for heavy electric vehicles. An HL tire supports more weight at the same inflation pressure than an XL tire. If your EV came with HL tires, replacements must also carry HL, a distinction covered in Michelin's XL and HL guidance (opens in a new tab).

What to do with a mismatch

Put your recorded specifications beside the current tire markings and compare each position. A completed check gives you the fitted size, service description and any special-load markings, plus the vehicle's requirements. You can then identify exactly which characters agree and which need a professional's attention.

Decoding the sidewall doesn't establish installation compatibility by itself. If you find a mismatch, ask a trained tire professional whether the existing size is compatible before replacing it with the same size; bring both records so the question is specific. Any proposed departure from the manufacturer's size also needs a compatibility assessment before installation.

There are practical reasons for that boundary. Changing overall diameter affects speedometer accuracy and wheel-well clearance, and can affect ABS and stability-control behavior. Changing width can alter handling balance, so a size change involves more than finding another tire with the same rim number.

Ask the professional to check the vehicle's placard, owner's manual and any applicable original-equipment technical bulletins against your findings. Those specifications account for the vehicle's load, steering, braking and stability requirements, including when only one or two tires are being replaced. Michelin's compatibility guidance (opens in a new tab) explains why the vehicle's own specifications govern that decision.