Tire Size Calculator & Comparison
Compare tire dimensions and calculate speedometer error
Current Tire
New Tire
Understanding Tire Size Calculations
When changing the wheels or tires on your vehicle, it is critical to understand how the new dimensions affect your car's performance, clearance, and speedometer accuracy. This tire size calculator allows you to compare two different tire setups side-by-side.
How to Read a Tire Sidewall
A standard tire size looks like 215/65R15. Here is what those numbers mean:
- 215 (Width): The width of the tire from sidewall to sidewall in millimeters.
- 65 (Aspect Ratio): The height of the sidewall expressed as a percentage of the width. In this case, the height is 65% of 215mm.
- 15 (Rim Diameter): The diameter of the wheel rim in inches.
Speedometer Error Explained
Your vehicle's speedometer is calibrated based on the factory tire's overall diameter. If you install a larger tire, the wheel travels further with each revolution. This means your speedometer will read slower than your actual speed. Conversely, a smaller tire will make the speedometer read faster than you are actually moving.
The 3% Rule
Most automotive experts recommend keeping the total diameter change within 3% of the original factory tire size. Exceeding this can lead to issues with brake performance, transmission shift points, and potential rubbing against the wheel well or suspension components.