This calculator and its associated content are verified for accuracy based on standard automotive engineering principles for tire sizing and speedometer calibration.
Changing tire sizes can significantly affect your vehicle’s performance, fuel economy, and crucially, your speedometer accuracy. Use this calculator to compare your original tire specifications with a new size to determine the exact difference in diameter, circumference, and speed.
Tire on Wheel Calculator
Calculation Summary
Detailed Calculation Steps
Tire on Wheel Calculator Formula
1. Sidewall Height (H):
$$H = \text{Section Width } (W) \times \frac{\text{Aspect Ratio } (R)}{100 \times 25.4}$$
2. Total Diameter (D) in Inches:
$$D = 2 \times H + \text{Rim Diameter } (D_{rim})$$
3. Circumference (C) in Inches:
$$C = D \times \pi$$
4. Speedometer Error (E):
$$E = \left( \frac{\text{New Diameter} – \text{Old Diameter}}{\text{Old Diameter}} \right) \times 100$$
Formula Source: Tire Rack Technical Specs | Goodyear Tire Information
Variables Explained
- Section Width (W): The width of the tire in millimeters, measured from the widest point of its outer sidewall to the widest point of its inner sidewall (e.g., 225).
- Aspect Ratio (R): The height of the sidewall as a percentage of the section width (e.g., 45% of 225mm).
- Rim Diameter (D): The diameter of the wheel rim in inches (e.g., 17 inches).
- Diameter/Circumference: The total height/distance traveled in one revolution of the tire.
- Speedometer Error: The percentage difference in wheel size, which directly translates to how far off your speedometer reading will be.
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What is a Tire on Wheel Calculator?
A Tire on Wheel Calculator, often called a Tire Size Comparison Calculator, is an essential tool for automotive enthusiasts and everyday drivers considering a change in tire size. It compares the geometric dimensions (diameter and circumference) of your original equipment manufacturer (OEM) tires with a potential replacement size.
The core purpose of this comparison is to calculate the impact on the vehicle’s gearing and, most critically, the accuracy of the speedometer. Since the speedometer relies on the vehicle’s computer measuring wheel rotations, changing the overall tire diameter directly alters the distance traveled per rotation, leading to a speed reading error.
A change in overall diameter of more than 3% is generally discouraged, as it can potentially cause issues with Anti-lock Braking Systems (ABS), Traction Control, and vehicle stability systems, in addition to significant speedometer inaccuracies.
How to Calculate Tire Diameter Difference (Example)
Let’s use the default example: Old Tire (225/45R17) vs. New Tire (245/40R18).
- Calculate Old Sidewall Height: $225 \times (45/100) = 101.25$ mm. Convert to inches: $101.25 / 25.4 \approx 3.986$ inches.
- Calculate Old Diameter: $17$ inches (Rim) $+ 2 \times 3.986$ inches (Sidewalls) $\approx 24.97$ inches.
- Calculate New Sidewall Height: $245 \times (40/100) = 98$ mm. Convert to inches: $98 / 25.4 \approx 3.858$ inches.
- Calculate New Diameter: $18$ inches (Rim) $+ 2 \times 3.858$ inches (Sidewalls) $\approx 25.716$ inches.
- Determine Diameter Change: The new tire is larger by $25.716 – 24.97 \approx 0.746$ inches.
- Calculate Speedometer Error: $\frac{0.746}{24.97} \times 100 \approx 2.99\%$. Since the new tire is larger, when your speedometer reads 60 MPH, your true speed is $60 \times 1.0299 \approx 61.79$ MPH.
Frequently Asked Questions (FAQ)
Most experts recommend keeping the overall change in tire diameter within a $\pm 3\%$ tolerance of the original size. Changes outside this range can severely impact speedometer accuracy, potentially cause rubbing against wheel wells, and strain transmission components.
Yes. Since the odometer measures distance based on the number of wheel revolutions, if the wheel diameter increases, your odometer will under-report the true distance traveled (and vice versa for a decrease in diameter).
The ‘R’ stands for Radial, indicating the internal construction of the tire. It is a mandatory part of the standard P-Metric tire code but is not used in the geometric calculation.
The total diameter is the rim diameter plus the height of the tire. Since a tire has a sidewall above the rim and a sidewall below the rim (when viewing the cross-section), you must multiply the calculated sidewall height by two.