How to Read a Tire Size
Upgrading your wheels or looking for a cheaper set of tires? Changing your tire size affects your car's clearance, gearing, and speedometer accuracy. Our Tire Size Calculator converts metric tire codes into understandable inches and shows you exactly how a new set will change your ride.
If you are tracking travel times and need to calculate precise hours driven based on your new speeds, check out our Hours Calculator.
Decoding the Sidewall
Most modern vehicles use metric tire sizing. For example, a 225/65R17 tire breaks down as follows:
- 225 (Width): The section width of the tire in millimeters from sidewall to sidewall.
- 65 (Aspect Ratio): The height of the sidewall, expressed as a percentage of the width. In this case, the sidewall is 65% of 225mm.
- R17 (Wheel Diameter): The "R" stands for Radial construction. The "17" is the diameter of the wheel (rim) in inches that the tire fits.
For a deeper dive into the history and meaning of these markings, visit the Wikipedia page on Tire Codes.
Plus Sizing & Speedometer Error
"Plus Sizing" refers to putting larger wheels on a car while reducing the tire's aspect ratio (sidewall height) to keep the overall diameter the same. This maintains speedometer accuracy and suspension clearance.
If you increase the overall diameter of the tire, your vehicle covers more ground per revolution. This means your speedometer will read slower than you are actually traveling, potentially leading to speeding tickets!
Golden Rule: Try to keep the new overall diameter within 3% of the factory tire diameter.