Tire Size Comparison Calculator
Use this calculator to compare the specifications of your current tires with a potential new tire size. Understanding the differences in overall diameter, sidewall height, and revolutions per mile can help you make informed decisions about speedometer accuracy, vehicle clearance, and handling characteristics.
Current Tire Specifications
The width of the tire in millimeters (e.g., 205 for 205/55R16).
The sidewall height as a percentage of the tire width (e.g., 55 for 205/55R16).
The diameter of the wheel in inches (e.g., 16 for 205/55R16).
New Tire Specifications
The width of the new tire in millimeters (e.g., 225 for 225/45R17).
The sidewall height as a percentage of the new tire width (e.g., 45 for 225/45R17).
The diameter of the new wheel in inches (e.g., 17 for 225/45R17).
Comparison Results
Current Tire Details
Sidewall Height: mm
Overall Diameter: mm ( inches)
Circumference: mm ( inches)
Revolutions per Mile:
Revolutions per Kilometer:
New Tire Details
Sidewall Height: mm
Overall Diameter: mm ( inches)
Circumference: mm ( inches)
Revolutions per Mile:
Revolutions per Kilometer:
Overall Comparison
Diameter Difference: mm ( inches)
Speedometer Difference:
Speedometer Example: When your speedometer reads 60 mph, your actual speed with the new tires will be approximately mph.
Understanding Tire Sizes and Their Impact
Tire sizes are typically displayed in a format like "205/55R16". Let's break down what these numbers mean:
- 205 (Tire Width): This is the width of the tire in millimeters, measured from sidewall to sidewall. A wider tire generally offers more grip but can affect fuel economy and steering feel.
- 55 (Aspect Ratio): This number represents the sidewall height as a percentage of the tire's width. In this example, the sidewall height is 55% of 205mm. A lower aspect ratio means a shorter sidewall, often found on performance tires for better handling and a stiffer ride.
- R (Construction Type): "R" stands for Radial, which is the most common type of tire construction today.
- 16 (Wheel Diameter): This is the diameter of the wheel (or rim) in inches that the tire is designed to fit.
Why Compare Tire Sizes?
Changing your tire size, even slightly, can have several significant impacts on your vehicle:
- Speedometer Accuracy: The most common concern. If the overall diameter of your new tires is different from your original tires, your speedometer will read incorrectly. A larger diameter tire will make your speedometer read lower than your actual speed, while a smaller diameter tire will make it read higher.
- Vehicle Clearance: Larger tires might rub against fender wells, suspension components, or other parts of your vehicle, especially during turns or over bumps.
- Handling and Ride Quality: Changes in sidewall height and tire width can alter your vehicle's handling characteristics. Shorter sidewalls (lower aspect ratio) typically provide sharper steering response but a firmer ride.
- Braking Performance: A larger contact patch from wider tires can improve braking, but the added rotational mass can also slightly increase stopping distances.
- Fuel Economy: Larger or wider tires can increase rolling resistance and rotational mass, potentially leading to a slight decrease in fuel efficiency.
- Gear Ratio Effect: A larger overall tire diameter effectively "lengthens" your vehicle's gear ratios, which can impact acceleration and towing performance.
How the Calculator Works
Our calculator takes the three key measurements for both your current and new tires and performs the following calculations:
- Sidewall Height: Calculated by multiplying the tire width by the aspect ratio percentage.
- Overall Tire Diameter: Determined by adding the wheel diameter (converted to millimeters) to twice the sidewall height.
- Tire Circumference: Calculated by multiplying the overall diameter by Pi (π).
- Revolutions per Mile/Kilometer: This indicates how many times the tire rotates to cover one mile or kilometer, derived from the tire's circumference.
- Speedometer Difference: This crucial metric shows the percentage difference in overall diameter between the new and current tires, directly translating to how much your speedometer will be off. It also provides an example of your actual speed when your speedometer reads a specific value (e.g., 60 mph).
Always consult with a professional tire installer or your vehicle manufacturer's specifications before making significant changes to your tire size.