How do I read a tire size?

A standard tire size like 225/55R17 means: 225 is the tire width in millimeters, 55 is the aspect ratio (the sidewall height is 55% of the width), R means it's a Radial tire, and 17 is the rim diameter in inches.

What happens if I change my tire size?

Changing your tire size can affect your vehicle's handling, fuel economy, and speedometer accuracy. A larger diameter tire will cause your speedometer to read slower than your actual speed, while a smaller tire will make it read faster.

How is tire diameter calculated?

The overall tire diameter is calculated by adding the rim diameter (converted to millimeters) to twice the sidewall height. The sidewall height is found by multiplying the tire width by the aspect ratio.

How does tire size affect the speedometer?

Your car's speedometer is calibrated to the original tire's circumference. If you install tires with a different circumference, the speedometer will be inaccurate. A larger tire covers more ground per revolution, making you travel faster than the speedometer indicates.

Can I fit wider tires on the same rim?

You can often fit slightly wider tires on the same rim, but there are limits. You must ensure the new tire width is approved for your rim width and that the tire doesn't rub against the suspension or fenders. Always consult a professional.

Tire Size Calculator — Compare Tire Dimensions and Fitment

🛞 Tire Size Calculator

Q: What is an acceptable tire size difference?

A general rule of thumb is to keep the overall diameter difference within 3% of the original tire size. Exceeding this can negatively impact your vehicle's braking system (ABS), traction control, and transmission shifting.

Original Tire

Width (mm)

Aspect Ratio (%)

Rim (in)

New Tire

Width (mm)

Aspect Ratio (%)

Rim (in)

Comparison Results

⚠️ **Warning:** The new tire's diameter is more than 3% different from the original. This may negatively affect your vehicle's braking system (ABS), traction control, transmission shift points, and fuel economy. Proceed with caution and consult a professional.

How to Read Tire Sizes

Understanding the numbers on your tire's sidewall is crucial for choosing the right replacement. Let's break down a common tire size, for example, 225/55R17:

225: This is the tire's width in millimeters, measured from sidewall to sidewall. This tire is 225 mm wide.
55: This is the aspect ratio. It means the height of the sidewall is 55% of the tire's width. A lower number indicates a shorter, sportier sidewall, while a higher number means a taller, more comfortable sidewall.
R: This stands for Radial construction, which is the standard for virtually all modern car tires.
17: This is the diameter of the wheel (or rim) that the tire is designed to fit, measured in inches.

Using these three key numbers, our calculator can determine all other important dimensions for a proper comparison.

Tire Size Calculator Explained

This tool is designed to help you see the exact dimensional differences between two tire sizes. Whether you're looking for a wider stance, a taller sidewall for comfort, or a different wheel size, it's vital to know how these changes will affect your vehicle. Our calculator provides a side-by-side comparison of the key metrics, including sidewall height, overall diameter, circumference, and how many times the tire will rotate over a kilometer.

Tire Diameter and Circumference

The overall diameter is one of the most critical measurements. It's the total height of the tire, from the ground to the top. A significant change in diameter can alter your vehicle's ride height, ground clearance, and fender gap. The circumference is directly related to the diameter (Circumference = Diameter × π) and determines how far the tire travels in one full rotation. This is the value that directly impacts your speedometer and odometer accuracy.

Speedometer Error and Tire Fitment

Your vehicle's computer, including the speedometer, ABS, and traction control systems, is calibrated for the original equipment (OE) tire size. When you change the tire circumference, you change the number of revolutions per kilometer. If your new tires are larger, they will rotate fewer times per kilometer, causing your speedometer to read lower than your actual speed. Conversely, smaller tires rotate more often, making your speedometer read higher. It's widely recommended to keep the diameter difference within a 3% tolerance to avoid significant issues with these critical systems.

FAQ — Tire Safety and Fitment

What is an acceptable tire size difference?
A diameter difference within 3% is generally considered safe. Anything beyond that can cause issues with safety systems like ABS and traction control, and may even affect your transmission's performance.

Can I put wider tires on my existing rims?
Every rim has a recommended range of tire widths it can safely accommodate. While a small increase is often possible, an excessively wide tire on a narrow rim can lead to poor handling and uneven wear. Always check the rim manufacturer's specifications.

Will bigger tires affect my fuel economy?
Yes. Larger, heavier tires increase rolling resistance and unsprung weight, which typically leads to a decrease in fuel efficiency.

What is "plus-sizing"?
Plus-sizing (e.g., "+1") means increasing your rim diameter by one inch while choosing a tire with a lower aspect ratio to keep the overall diameter nearly the same as the original. For example, moving from a 16-inch wheel to a 17-inch wheel.