1. What is a Tire Size and Speedometer Calculator?

Definition: This calculator determines how changing your tire size affects your actual speed compared to what your speedometer shows.

Purpose: It helps vehicle owners and mechanics understand speedometer accuracy after tire size modifications.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( S_{new} \) — Actual speed with new tires (mph)
• \( S_{old} \) — Speedometer reading (mph)
• \( TD_{new} \) — Diameter of new tires (inches)
• \( TD_{old} \) — Diameter of original tires (inches)

Explanation: Larger tires cover more ground per rotation, making your actual speed faster than what's shown. Smaller tires have the opposite effect.

3. Importance of Speedometer Calculation

Details: Correct calculations prevent speeding tickets, ensure proper transmission shifting, and maintain accurate odometer readings.

4. Using the Calculator

Tips: Enter your speedometer reading, original tire diameter, and new tire diameter. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: Where can I find my tire diameter?
A: Check tire manufacturer specs or use online tire size calculators that convert size markings (e.g., 225/45R17) to diameter.

Q2: How much difference is significant?
A: A 3% or greater difference typically requires speedometer recalibration to maintain accuracy.

Q3: Does this affect my odometer?
A: Yes, the same ratio applies - larger tires will cause your odometer to under-report actual mileage.

Q4: What if I change both width and aspect ratio?
A: The calculation only needs the overall diameter change, regardless of how you achieve it.

Q5: How do I correct my speedometer?
A: Many modern vehicles can be recalibrated via OBD-II tools; others may need a speedometer gear change or recalibration module.