How to Use the Tire Size Calculator
The tire size calculator is an essential tool for automotive enthusiasts, mechanics, and everyday drivers looking to upgrade their wheels or tires. Understanding how a new tire size compares to your factory equipment is crucial for maintaining speedometer accuracy, ground clearance, and proper gearing.
To use this calculator, follow these simple steps:
- Tire Width
- The first number in a tire sequence (e.g., 215). It represents the width of the tire from sidewall to sidewall in millimeters.
- Aspect Ratio
- The two-digit number following the slash (e.g., 65). This is the height of the sidewall expressed as a percentage of the width.
- Rim Diameter
- The number following the 'R' (e.g., 15). This is the diameter of the wheel in inches that the tire is designed to fit.
How the Math Works
Calculating tire dimensions involves converting millimeters to inches and using the relationship between diameter and circumference. The total diameter of a tire is the sum of the wheel diameter plus the height of two sidewalls.
Total Diameter = Rim Diameter + 2 × (Width × Aspect Ratio / 100 / 25.4)
- Sidewall Height: Calculated by multiplying the width by the aspect ratio. We divide by 25.4 to convert millimeters to inches.
- Circumference: Diameter × π (3.14159…). This tells you how far the vehicle travels in one full rotation.
- Speedometer Difference: Since speedometers count tire revolutions, a larger tire covers more ground per revolution, making the speedometer read lower than your actual speed.
Tire Size Calculation Example
Scenario: You are upgrading from a standard 215/65R15 tire to a more aggressive 225/60R16 setup.
Step-by-step solution for the Current Tire:
- Width = 215mm
- Sidewall Height = (215 * 0.65) / 25.4 = 5.50 inches
- Total Diameter = (5.50 * 2) + 15 = 26.00 inches
- Circumference = 26.00 * 3.14159 = 81.68 inches
- Revolutions per Mile = 63,360 / 81.68 = 776 revs/mile
Common Questions
Is a 3% difference in tire size okay?
Generally, most experts recommend staying within 3% of the original tire diameter. Exceeding this can cause issues with Anti-lock Braking Systems (ABS), Electronic Stability Control (ESC), and may lead to the tire rubbing against the fender or suspension components.
How does width affect my vehicle?
A wider tire increases the contact patch with the road, often improving grip and handling. However, it also increases rolling resistance, which can slightly decrease fuel economy and may lead to increased road noise.
What if I change my rim size?
When increasing rim size (e.g., going from 17″ to 19″), you must decrease the aspect ratio (the tire profile) to keep the overall diameter similar. This is often called "Plus Sizing."