The correct tire size is crucial for a vehicle’s safety and handling characteristics. Manufacturers engineer a vehicle’s suspension, braking, and electronic systems to function optimally with a specific tire size and construction. Using the wrong size can compromise the vehicle’s operation and may lead to premature wear of components. Understanding how to identify and interpret the manufacturer’s recommended specifications ensures your vehicle operates as designed.
Where to Find the Recommended Size
The primary source for your vehicle’s tire size is the information placard provided by the manufacturer. This sticker is typically located on the driver’s side door jamb. The placard lists the original equipment tire size, the correct inflation pressures for both front and rear tires, and the vehicle’s maximum load capacity.
The owner’s manual, usually found in the glove compartment, is another accurate source for the recommended specifications. This information may also be printed on the inside of the fuel filler door or within the glove box itself. While the size is molded into the sidewall of the tires currently on your car, always defer to the manufacturer’s placard or manual to confirm the original equipment size, as a previous owner may have installed an incorrect size.
Decoding the Tire Size Code
The tire size is communicated through a standardized sequence of letters and numbers, such as P205/55R16. The first letter, P, designates the tire type, with “P” standing for P-metric, the standard for passenger vehicles. Other common designations include “LT” for light trucks.
The three-digit number that follows, 205, indicates the tire’s section width, measured in millimeters from one sidewall to the other. This measurement is a nominal width. The number after the slash, 55, is the aspect ratio.
The aspect ratio represents the height of the tire’s sidewall as a percentage of its width; in the example, the sidewall height is 55% of the 205-millimeter width. The letter R signifies the tire’s internal construction, standing for radial, the method used in nearly all modern passenger car tires. Finally, the number 16 indicates the diameter of the wheel rim in inches that the tire is designed to fit.
Safety Ratings: Load and Speed
Following the main size code on the sidewall are two additional characters representing the tire’s service description: the Load Index and the Speed Rating. These ratings are separate from the physical dimensions and relate directly to the tire’s safety performance. The Load Index is a numerical code corresponding to the maximum weight that a single tire can safely support when properly inflated.
The replacement tire’s Load Index must meet or exceed the specification set by the manufacturer. Using a tire with a lower load index means it cannot support the vehicle’s maximum weight, which can lead to tire failure. The Speed Rating is represented by a letter (e.g., H, V, W) and indicates the maximum speed the tire is rated to sustain under its approved load capacity.
This rating is a safety measure related to heat generation and structural integrity, not a recommended driving speed. The replacement tire’s speed rating must be equal to or higher than the original equipment rating to maintain the vehicle’s engineered performance characteristics.
The Consequences of Changing Tire Size
Deviating from the manufacturer’s recommended overall tire diameter negatively affects the vehicle’s operation and safety systems. One immediate effect is an error in the speedometer and odometer readings. The vehicle’s computer is calibrated based on the original tire’s circumference, so a larger tire rotates fewer times per mile, causing the speedometer to display a speed slower than the actual speed.
A change in overall diameter also affects the calibration of electronic safety systems, such as the Anti-lock Braking System (ABS) and Traction Control. These systems rely on accurate wheel speed data, and an incorrect tire size can disrupt this data, potentially leading to system malfunction. Installing a significantly larger tire risks physical clearance issues, where the tire may rub against the wheel well, suspension components, or fender.
A general guideline is to avoid a variance of more than 3% in the overall diameter from the original equipment size. Any change in tire size should be carefully calculated to maintain the original rolling diameter, often by adjusting the rim size and aspect ratio simultaneously. Consulting a tire professional is advisable before making any size change to ensure compatibility with the vehicle’s engineering and safety requirements.