Tire Size Calculator

What Is a Tire Size Calculator?

A tire size calculator is a tool that compares two tire sizes and shows how they differ in:

• Overall tire height
• Sidewall height
• Circumference
• Revolutions per mile
• Speedometer accuracy
• Ride height and ground clearance

Instead of relying on rough estimates, the calculator uses math based on tire specifications to give precise results. This is especially useful when upgrading wheels, changing tire widths, or adjusting aspect ratios.

Understanding Tire Size Numbers (245/45R18 Example)

Before using a calculator, it helps to know what the numbers mean.

245/45R18 breaks down like this:

• 245 – Tire width in millimeters
• 45 – Aspect ratio (sidewall height as a percentage of width)
• R – Radial construction
• 18 – Wheel diameter in inches

The calculator uses these values to build the full tire dimensions.

What This Tire Size Calculator Does

The calculator you are using compares a current tire (baseline) with a new tire (comparison) and then shows how the change affects your vehicle.

Here is what it calculates and why each value matters.

1. Tire Height

This is the total diameter of the tire from top to bottom.

Why it matters:

• A taller tire raises the car
• A shorter tire lowers the car
• Changes affect suspension geometry and handling

2. Sidewall Height

Sidewall height affects comfort and responsiveness.

• Taller sidewalls absorb bumps better
• Shorter sidewalls feel firmer and respond faster

The calculator converts millimeters to inches so the result is easy to visualize.

3. Circumference

Circumference is the distance the tire travels in one full rotation.

Why it matters:

• A larger circumference travels farther per rotation
• This directly affects speedometer accuracy

4. Revolutions Per Mile

This tells you how many times the tire rotates over one mile.

• Smaller tires = more revolutions
• Larger tires = fewer revolutions

This value matters for ABS, traction control, and transmission shift timing.

5. Speedometer Error

Your speedometer assumes a specific tire size. Change the tire height, and the speed reading changes too.

Example:

• Speedometer shows 65 MPH
• Actual speed might be 67 MPH or 63 MPH

The calculator shows:

• Percentage error
• Actual speed at 65 MPH

This is one of the most important safety-related outputs.

6. Ride Height and Clearance Change

The calculator shows how much higher or lower your car will sit.

• The value is split in half because only half the tire affects ride height
• Even a 0.5-inch change can matter for clearance

This helps you avoid rubbing issues with fenders, suspension, or wheel wells.

Why the ±3% Rule Matters

Industry guidelines recommend staying within ±3% tire diameter difference.

This calculator clearly flags that threshold:

• Within ±1.5% – Usually safe, minimal impact
• 1.5% to 3% – Noticeable changes, caution advised
• Over 3% – Risk of ABS errors, traction control issues, and shifting problems

The built-in warning system is there to prevent costly mistakes.

Real-World Accuracy: The 3% Load Factor

Unlike basic calculators, this one applies a 3% static load factor when calculating revolutions per mile.

Why this matters:

• Tires flatten slightly under vehicle weight
• Real rolling radius is smaller than theoretical size
• Results better reflect real driving conditions

This small detail improves accuracy in speed and rev calculations.

When Should You Use a Tire Size Calculator?

A tire size calculator is useful when:

• Upsizing or downsizing wheels
• Changing tire width or aspect ratio
• Planning a staggered setup
• Comparing winter and summer tire sizes
• Checking speedometer accuracy after a change

If the tire size is different from stock, you should run the numbers first.

Common Mistakes This Calculator Helps You Avoid

• Choosing tires that throw off speed readings
• Causing ABS or traction control faults
• Creating clearance or rubbing issues
• Assuming wider tires are always better
• Ignoring sidewall height changes

A few minutes with a calculator can save money and frustration later.

How to Use the Results Correctly

Do not focus on one number alone. Look at the full picture:

• Diameter difference
• Speedometer error
• Clearance change
• Warnings provided

If the calculator shows a warning, take it seriously. Small differences on paper can feel much bigger on the road.