Scale to GramsHow to Read Tire Size: What the Numbers on Your Tire Mean
The Tire Size Format Explained
A typical tire size reads something like P225/45R17 91V. This looks like random characters, but each part conveys specific information. The letter prefix (P) indicates the tire type — P means passenger vehicle, LT means light truck, ST means special trailer. The first number (225) is the tire width in millimeters, measured from sidewall to sidewall. The second number after the slash (45) is the aspect ratio — the height of the sidewall expressed as a percentage of the width. The letter R means radial construction (virtually all modern tires are radial). The number after R (17) is the rim diameter in inches — this must match your wheel size. The number after a space (91) is the load index — how much weight the tire can support. The final letter (V) is the speed rating — the maximum safe speed.
Width: The First Number
The tire width (225 in our example) is measured in millimeters from one sidewall to the other when the tire is mounted and properly inflated. Common widths range from 155mm for small economy cars to 315mm or more for sports cars and SUVs. Wider tires provide more grip because they have a larger contact patch with the road. Narrower tires are more fuel-efficient and perform better in snow because they concentrate the vehicle's weight into a smaller area, cutting through slush to reach the pavement. When replacing tires, staying within 10-20mm of your original width is generally safe without other modifications. Going wider than that may cause rubbing on wheel wells or affect handling.
Aspect Ratio: The Second Number
The aspect ratio (45 in our example) tells you the sidewall height as a percentage of the width. A 225/45 tire has a sidewall height of 225 x 0.45 = 101.25mm. Lower aspect ratios (35-45) mean shorter sidewalls, which gives sharper steering response and a sportier look but a firmer ride. Higher aspect ratios (55-70) mean taller sidewalls, which provides a more comfortable ride and better pothole protection but less precise handling. The aspect ratio directly affects the overall tire diameter. Changing aspect ratios without adjusting rim size changes your speedometer accuracy, ride height, and can affect safety systems like ABS and traction control. Always replace tires with the same aspect ratio unless you are also changing wheel size.
Rim Size, Load Index, and Speed Rating
The rim diameter (17 in our example) must match your wheel exactly. You cannot put a 17-inch tire on a 16-inch wheel. Common rim sizes for passenger cars range from 14 to 22 inches, with 16-18 being the most common. The load index (91 in our example) corresponds to the maximum weight each tire can support. A load index of 91 means 615 kg (1,356 lbs) per tire. Multiply by four for the total vehicle capacity. You should never install tires with a lower load index than your vehicle requires — check your owner's manual or the sticker on the driver's door jamb. The speed rating letter indicates the maximum sustained speed. Common ratings: S = 112 mph, T = 118 mph, H = 130 mph, V = 149 mph, W = 168 mph, Y = 186 mph. Your replacement tires should match or exceed the original speed rating.
Finding Your Tire Size
The easiest way to find your current tire size is to look at the sidewall of your tire — the numbers are molded into the rubber in raised lettering. If the tires are dirty, the numbers can be hard to read. In that case, check the tire information placard on the inside of the driver's door jamb, which lists the original equipment tire size. Your owner's manual also lists the correct tire sizes for your vehicle, often with multiple approved sizes for different trim levels or wheel options. You can also photograph your tire sidewall and use an AI tool like Scale to Grams to automatically read and decode the tire size numbers from the photo — especially useful when the embossed numbers are hard to read in person.
When Tire Sizes Matter Most
Getting the right tire size is critical when buying replacement tires. Incorrect sizing can affect speedometer accuracy (each size change shifts readings), ride height and ground clearance, braking distance, fuel economy, and the proper functioning of ABS, traction control, and stability systems. Mismatched tire sizes between axles (different front and rear sizes that are not specified by the manufacturer) can cause drivetrain damage on all-wheel-drive vehicles. If you are considering changing tire sizes for aesthetics or performance (wider tires, lower profile), consult a tire specialist to ensure the new size is compatible with your wheels, wheel wells, and vehicle systems.
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