Changed your tire size? Calculate your true speed and find your speedometer's error percentage.
The code on your tire's sidewall provides all the information needed. Let's break down an example: P225/45R17
P stands for "P-Metric," indicating a passenger car tire. You might also see LT for Light Truck.
This is the tire's width from sidewall to sidewall, measured in millimeters. In this case, 225mm.
This is the height of the sidewall as a percentage of the tire's width. Here, the sidewall height is 45% of 225mm.
R stands for "Radial," which is the most common type of tire construction today.
This is the diameter of the wheel that the tire is designed to fit on, measured in inches.
An inaccurate speedometer isn't just a minor inconvenience; it can have significant safety and legal consequences.
If your new tires are larger, your actual speed will be higher than your speedometer indicates. This can lead to unintentional speeding and costly tickets.
Modern safety systems like Anti-lock Brakes (ABS) and Electronic Stability Control (ESC) rely on accurate wheel speed data. A significant error can affect their performance in critical situations.
Your odometer reading will also be incorrect, affecting your vehicle's resale value and your ability to track maintenance intervals accurately.
Changing tire size effectively changes your vehicle's final drive ratio. This can impact acceleration, fuel economy, and cause premature wear on transmission components.