Use this Wheel and Tire Calculator to compare the specifications, overall diameter, and speed changes between your current tire setup and a potential new tire size. Enter the three primary values for both the old and new tires to get started.
Wheel and Tire Calculator
Current Tire (Pxxx/xxRxx)
New Tire (Pxxx/xxRxx)
Wheel and Tire Calculator Formula
The core formula for determining the overall diameter of a tire involves converting the sidewall height (which is a percentage of the width) to inches and adding the rim diameter.
Overall Diameter (OD) in inches = (SH in inches * 2) + Rim Diameter (in)
Diameter Difference (%) = ((OD New – OD Old) / OD Old) * 100
Variables
- Tire Width (mm): The section width of the tire in millimeters (the first number in the tire size, e.g., 225).
- Aspect Ratio (%): The ratio of the tire’s sidewall height to its width, expressed as a percentage (the second number, e.g., 45).
- Rim Diameter (in): The diameter of the wheel (rim) in inches (the third number, e.g., 17).
- Overall Diameter (OD): The total diameter of the inflated tire/wheel assembly.
- Speedometer Error: The percentage difference in diameter directly translates to the percentage difference in speed readings.
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What is a Wheel and Tire Calculator?
A wheel and tire calculator is a crucial online tool for enthusiasts and professionals who plan to change their vehicle’s wheel or tire size. Tires are specified using a standardized code (e.g., 225/45R17) where each number represents a critical dimension. The calculator translates these dimensions into key physical metrics like overall diameter and circumference, enabling users to understand the fitment consequences before making a purchase.
The most important output of this calculator is the ‘Speedometer Error.’ The vehicle’s onboard computer uses the original tire’s circumference to determine how many rotations equal a mile. If a new tire has a different circumference, the computer’s speed calculation will be incorrect. This tool quantifies that error, allowing drivers to make safe and legal modifications.
How to Calculate Wheel and Tire Diameter (Example)
- Identify the Size: Take a tire size, for example, 245/40R18. The width (W) is 245 mm, the aspect ratio (AR) is 40%, and the rim diameter (D) is 18 inches.
- Calculate Sidewall Height in mm: Multiply the width by the aspect ratio: $245 \text{ mm} \times (40 / 100) = 98 \text{ mm}$.
- Convert Sidewall Height to Inches: Divide the result by $25.4$: $98 \text{ mm} / 25.4 \approx 3.858 \text{ inches}$.
- Calculate Overall Diameter: The overall diameter is the rim diameter plus two sidewall heights: $18 \text{ inches} + (2 \times 3.858 \text{ inches}) \approx 25.716 \text{ inches}$.
- Compare and Correct: Compare this new diameter to your old tire’s diameter. If the new diameter is 3% larger than the old one, your speedometer will read 60 MPH when your actual speed is $60 \times 1.03 = 61.8 \text{ MPH}$.
Frequently Asked Questions (FAQ)
- Is it safe to install tires with a different diameter? It is generally recommended to keep the diameter difference under $\pm 3\%$. Differences greater than this can cause issues with your speedometer, ABS, traction control, and transmission shifting points.
- What is the difference between overall diameter and circumference? Overall diameter is the distance across the tire through the center, while circumference is the distance around the tire (Diameter $\times \pi$). Circumference is what dictates the speedometer reading.
- Does changing the rim diameter (R) affect the overall tire diameter? Yes, but only if you do not adjust the aspect ratio (AR) and tire width (W). To keep the overall diameter the same when increasing the rim size, you typically need to decrease the aspect ratio (a “low profile” tire).
- Why does my speedometer read lower than my actual speed? This happens if your new tire’s overall diameter is larger than the original factory tire. The tire is covering more ground per rotation, making you travel faster than the car computer thinks.