Vehicle Utilities

Tire Size Calculator

Compare the dimensions of two different tire specifications side-by-side and find speed reading changes.

Tire 1 Specification (Baseline)

Section Width

Aspect Ratio

Wheel Diam.

Tire 2 Specification (Alternative)

Section Width

Aspect Ratio

Wheel Diam.

Enter valid specifications to compare tire dimensions.

Vehicle Engineering

Understanding
Tire Dimensions

Car tires are specified by a series of numbers (e.g. 245/40R18) printed on the sidewall. Understanding these numbers is crucial when choosing alternative sizes to prevent mechanical issues or speedometer errors.

Width & Aspect

Width dictates the tire contact patch. Aspect ratio measures the sidewall profile as a percentage of tread width.

Diameter Match

Keeping the total wheel diameter close to original specification prevents speed reading errors and body rubbing.

Safety Limits

It is generally advised to keep tire diameter changes within ±3% of baseline stock sizes for safety and electronics stability.

Breaking Down the Tire Code

Let's decode a standard tire specification such as 245/40R18:

245 (Section Width): The width of the tire tread in millimeters (245 mm).
40 (Aspect Ratio): The height of the sidewall expressed as a percentage of the width. Here, the sidewall height is 40% of 245 mm (98 mm).
R (Radial Construction): Indicates the ply cords run radially across the tire casing.
18 (Wheel Diameter): The diameter of the wheel rim in inches.

Vehicle Safety Best Practices:

• Clearance Checks: Larger widths might rub against suspension struts or fender linings when turning full lock.
• Load Index: Ensure replacement tires equal or exceed the manufacturer's load carrying index.
• Speed Rating: Always use tires that meet the maximum potential speed capability of the vehicle.