Tire Size Comparison Calculator

How to Calculate Tire Size Comparison

Comparing tire sizes is crucial when considering changing your vehicle's tires. This calculator helps you understand the differences between two tire sizes in terms of overall diameter, circumference, width, sidewall height, and revolutions per mile. It also calculates the potential speedometer error that may result from changing tire sizes.

Formulas for Tire Size Comparison

The following formulas are used in our tire size comparison calculations:

1. Sidewall height (in inches): \[Sidewall = \frac{Width \times Aspect}{2540}\]
2. Overall diameter (in inches): \[Diameter = Rim Diameter + (2 \times Sidewall)\]
3. Circumference (in inches): \[Circumference = Diameter \times \pi\]
4. Revolutions per mile: \[Revs/Mile = \frac{63360}{Circumference}\]
5. Speedometer error (%): \[Error = (\frac{Revs/Mile_{old}}{Revs/Mile_{new}} - 1) \times 100\]

Calculation Steps

1. Calculate the sidewall height for both tires using the width and aspect ratio.
2. Determine the overall diameter by adding the rim diameter to twice the sidewall height.
3. Calculate the circumference of each tire using the overall diameter.
4. Compute the revolutions per mile for each tire size.
5. Calculate the difference between the two tire sizes for each dimension.
6. Determine the speedometer error percentage based on the change in revolutions per mile.

Example Calculation

Let's compare two tire sizes: 225/45R17 and 235/40R18

Tire One (225/45R17):

1. Sidewall height: \[\frac{225 \times 45}{2540} = 3.99 \text{ inches}\]
2. Overall diameter: \[17 + (2 \times 3.99) = 24.98 \text{ inches}\]
3. Circumference: \[24.98 \times \pi = 78.46 \text{ inches}\]
4. Revolutions per mile: \[\frac{63360}{78.46} = 807.55\]

Tire Two (235/40R18):

1. Sidewall height: \[\frac{235 \times 40}{2540} = 3.70 \text{ inches}\]
2. Overall diameter: \[18 + (2 \times 3.70) = 25.40 \text{ inches}\]
3. Circumference: \[25.40 \times \pi = 79.79 \text{ inches}\]
4. Revolutions per mile: \[\frac{63360}{79.79} = 793.96\]

Speedometer Error:

\[Error = (\frac{807.55}{793.96} - 1) \times 100 = 1.71\%\]

This means that with the new tires, the vehicle will actually be going 1.71% faster than what the speedometer indicates.

Tire Size Comparison Diagram

This diagram visually represents the differences between the two tire sizes across various dimensions. It helps to easily compare the overall diameter, circumference, width, sidewall height, and revolutions per mile for both tire sizes.