Free Tire Size Calculator

A tire size comparison calculator is an indispensable tool for anyone looking to understand, replace, or upgrade their vehicle’s tires. This tire diameter calculator and tire sidewall height calculator delivers precise measurements—diameter, sidewall height, and circumference—based on the alphanumeric code printed on the tire sidewall. It also functions as a tire code decoder, translating cryptic markings into actionable numbers, and as a tire circumference calculator that tells you how many revolutions a tire makes per mile. Most importantly, its tire size comparison feature lets you evaluate a prospective tire against your current one, predicting fitment issues and speedometer changes before you buy.

Understanding the Tire Code

Every tire carries a code that reveals its size and construction. The system is standardized, and once you know the pattern, you can extract all the dimensions needed for calculation.

• Vehicle class letter (optional): P (passenger), LT (light truck), ST (special trailer), T (temporary spare). If no letter is present, a diagonal-ply construction is assumed.
• Section width: A three-digit number indicating the nominal width in millimeters, measured at the widest sidewall point.
• Aspect ratio: The two-digit figure following a slash represents the sidewall height as a percentage of the width.
• Construction code (optional): R for radial (more than 98% of modern tires), D for diagonal (cross-ply), B for bias belt. If omitted, diagonal is assumed.
• Wheel diameter: The final one‑ or two‑digit number in inches (rarely three digits indicating millimeters).

For example, in the code P215/65R15, the “215” is the width in mm, “65” is the aspect ratio, “R” means radial, and “15” is the rim diameter in inches.

Computing Key Tire Dimensions

With the code decoded, you can calculate the three primary measures.

Sidewall height ($h$):

$$h = \frac{\text{aspect ratio} \times \text{width}}{100}$$

Using the P215/65R15 tire: $h = \frac{65 \times 215}{100} = 139.75\ \text{mm}$. Converted to inches, $h \approx 139.75 / 25.4 \approx 5.5\ \text{in}$.

Tire diameter ($D$):

$$D = d_{\text{wheel}} + 2h$$

With a 15‑inch wheel: $D = 15 + 2 \times 5.5 = 26\ \text{in}$.

Circumference ($C$):

$$C = \pi \times D \approx 3.1416 \times 26 = 81.7\ \text{in}$$

Revolutions per mile ($N$):

$$N = \frac{63360}{C} \approx \frac{63360}{81.7} \approx 776\ \text{rev/mi}$$

Measurement	Value	Unit
Sidewall height	139.75	mm
Sidewall height	5.5	in
Wheel diameter	15	in
Tire diameter	26	in
Circumference	81.7	in
Revolutions per mile	776	rev/mi

These values are the foundation for comparing tires and understanding how a change in size affects vehicle behavior.

How to Use the Tire Size Calculator

The calculator streamlines the above process and adds a comparison capability.

1. Enter the width, aspect ratio, and wheel diameter obtained from the tire code. Optionally, select the construction type (radial, diagonal, etc.), though it does not alter the calculated dimensions.
2. The tool instantly displays the tire diameter, sidewall height, circumference, and revolutions per mile. You can switch between imperial and metric units at any time.
3. To compare a second tire, tick the “Compare with another tire” box. Enter the second tire’s dimensions or code, and the calculator shows both sets of results side by side.
4. The comparison output includes the absolute diameter difference and the percentage change. A change greater than ±2.5% indicates that the new tire may not fit properly. Too large a tire could rub against the wheel arch, while too small a tire reduces ground clearance and may cause bottoming out.
5. The calculator is also a speedometer adjuster. Input a speedometer reading, and it displays the actual speed given the tire diameter change. This helps you stay within legal limits and adjust your driving habits accordingly.

Practical Example: 195/65R15 vs. 195/85R15

Suppose you are considering moving from a 195/65R15 tire to a 195/85R15. Entering both into the comparison feature yields the following:

• First tire (195/65R15): diameter ≈ 25.0 in.
• Second tire (195/85R15): diameter ≈ 28.1 in.
• Absolute difference: 3.07 in.
• Percentage change: $\frac{3.07}{25.0} \times 100\% \approx 12.3\%$, far exceeding the recommended 2.5% threshold.

Such a drastic change would likely cause clearance problems and significantly alter the speedometer. If the speedometer indicates 30 mph, the actual speed becomes:

$$\text{Actual speed} = 30 \times (1 + 0.123) \approx 33.7\ \text{mph}$$

This discrepancy could lead to speeding fines and inaccurate odometer readings. Modern vehicles often allow recalibration through the onboard electronics; a professional mechanic can assist with this adjustment.

Whether you are a DIY enthusiast or a professional technician, a tire size calculator equipped with comparison and decoder functions helps you make informed decisions. By precisely computing diameter, sidewall height, circumference, and their impact on speedometer accuracy, you can ensure that any new tire fits your vehicle safely and performs as expected.