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Speed Ratio Formula:
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The Tire Size To Speed Ratio calculates how a change in tire diameter affects vehicle speed. When you change tire sizes, your speedometer reading may no longer be accurate because larger or smaller tires rotate at different rates to cover the same distance.
The calculator uses the simple ratio formula:
Where:
Explanation: A ratio greater than 1 means your actual speed is higher than what's shown on the speedometer. A ratio less than 1 means your actual speed is lower than indicated.
Details: Calculating this ratio is essential for maintaining accurate speed readings, ensuring proper vehicle performance, and avoiding speeding tickets when changing tire sizes. It also helps in selecting appropriate gear ratios for optimal vehicle performance.
Tips: Measure both tire diameters accurately in inches. Enter the values in the corresponding fields. The calculator will compute the ratio which you can use to determine how much your speedometer reading differs from your actual speed.
Q1: How do I measure tire diameter accurately?
A: The most accurate method is to measure the mounted tire's height from the ground to the top of the tire while the vehicle is on a level surface with proper inflation.
Q2: What is considered a significant ratio difference?
A: Generally, a ratio difference of more than 3% (0.97-1.03) may require speedometer recalibration to maintain accuracy.
Q3: Does this ratio affect odometer readings?
A: Yes, the same ratio applies to odometer readings. Larger tires will cause the odometer to record fewer miles than actually traveled.
Q4: Can I use this calculator for metric tire sizes?
A: Yes, but you must first convert metric measurements to inches for accurate results, or ensure both measurements are in the same unit system.
Q5: How does tire width affect the speed ratio?
A: Tire width doesn't directly affect the speed ratio calculation. The ratio is based on tire diameter, which is the critical measurement for rotational distance.