How to Calculate Speedometer Error
Calculating speedometer error due to tire size changes involves comparing the diameter of the original tires to the new tires. Here's a comprehensive guide on how to use this calculator and understand the calculations behind it.
The basic formula used for calculating the actual speed is:
\[Actual Speed = Indicated Speed \times \frac{New Tire Diameter}{Original Tire Diameter}\]
Where:
- Actual Speed: The true speed of the vehicle
- Indicated Speed: The speed shown on the speedometer
- New Tire Diameter: The diameter of the new tires
- Original Tire Diameter: The diameter of the original tires
The tire diameter is calculated using:
\[Tire Diameter = \frac{(Width \times (Aspect / 100) \times 2 + Rim \times 25.4)}{25.4}\]
Calculation Steps
- Calculate the diameter of the original tires using the tire size formula.
- Calculate the diameter of the new tires using the tire size formula.
- Divide the new tire diameter by the original tire diameter to get the ratio.
- Multiply the indicated speed by this ratio to get the actual speed.
Example Calculation
Let's calculate the speedometer error for the following scenario:
- Original Tire Size: 205/55R16
- New Tire Size: 225/50R17
- Indicated Speed: 60 mph
- Original tire diameter: (205 * (55/100) * 2 + 16 * 25.4) / 25.4 ≈ 24.88 inches
- New tire diameter: (225 * (50/100) * 2 + 17 * 25.4) / 25.4 ≈ 25.67 inches
- Diameter ratio: 25.67 / 24.88 ≈ 1.0317
- Actual speed: 60 mph * 1.0317 ≈ 61.90 mph
Therefore, when the speedometer shows 60 mph, the actual speed is about 61.90 mph, an error of approximately 3.17%.
Speedometer Error Diagram