Tire Size Calculator

What This Calculator Does

The Tire Size Calculator is an easy-to-use tool designed to help you compare two different tire sizes with precision and clarity. By entering the width, aspect ratio, and rim diameter for both your current and prospective tires, you can instantly see detailed measurements like overall diameter, sidewall height, circumference, revolutions per mile, and how these differences can affect your speedometer reading. This calculator is invaluable for anyone considering a tire size change, ensuring your choice is safe, compatible, and meets your driving needs.

How to Use This Calculator

1. Enter the width, aspect ratio, and rim diameter for your current tire in the first set of input fields (Width (mm), Aspect Ratio (%), Rim Diameter (inches)).
2. Enter the same three values for your new or alternative tire in the second set of input fields.
3. Review the instant comparison of both tires, including detailed dimensions (such as overall diameter, sidewall height, and circumference) for each.
4. Examine the diameter difference and the calculated speedometer error to understand how a new tire size may affect your driving speed accuracy.
5. Use the results to make an informed decision about compatibility, safety, and performance before purchasing or installing new tires.

Definitions of Key Terms

Width (mm)

The section width of the tire, measured in millimeters from sidewall to sidewall. This is the first number in the tire size code (e.g., 225/45R17).

Aspect Ratio (%)

The ratio of the tire's sidewall height to its width, expressed as a percentage. For example, if the aspect ratio is 45, the sidewall height is 45% of the tire's width.

Rim Diameter (inches)

The diameter of the wheel rim, measured in inches. This is the third number in the tire size code (e.g., 225/45R17).

Size

The full tire size specification (e.g., 225/45R17), summarizing width, aspect ratio, and rim diameter.

Overall Diameter

The total height of the tire from the ground to the top of the tread, including both sidewalls and the rim.

Sidewall

The height of the tire’s sidewall, measured from the rim to the outer tread. This is calculated from the width and aspect ratio.

Circumference

The distance around the outer edge of the tire, measured in inches or millimeters. This affects how far your car travels with each tire revolution.

Revs per Mile

The number of times the tire rotates in one mile of travel. It’s influenced by the tire’s circumference and impacts speedometer readings.

Diameter Difference

The measurement difference in overall diameter between your current and new tires, usually shown in millimeters, inches, and percentage. This helps assess compatibility.

Speedometer Error

The calculated percentage difference in speedometer reading due to a change in tire circumference. It shows whether your speedometer will over- or under-report your actual speed.

Calculation Methodology

Sidewall Height (mm) = (Width (mm) × Aspect Ratio (%)) ÷ 100

Overall Diameter (mm) = (2 × Sidewall Height) + (Rim Diameter (inches) × 25.4)

Overall Diameter (inches) = Overall Diameter (mm) ÷ 25.4

Circumference (mm) = Overall Diameter (mm) × π

Circumference (inches) = Overall Diameter (inches) × π

Revs per Mile = 63,360 ÷ Circumference (inches)

Diameter Difference = New Overall Diameter (mm) - Original Overall Diameter (mm)

Diameter Difference (%) = (Diameter Difference ÷ Original Overall Diameter (mm)) × 100

Speedometer Error (%) = (New Circumference ÷ Original Circumference) × 100 - 100

These calculations use standard tire geometry formulas. Width and aspect ratio determine sidewall height, which, combined with rim diameter, gives you the overall diameter. Circumference and revolutions per mile are derived from the overall diameter, while differences between new and original tire sizes show potential impacts on fitment and speedometer readings.

Practical Scenarios

• Upgrading to Larger Wheels: If you want to switch from 16-inch to 18-inch wheels, the calculator helps you find tire profiles that maintain a similar overall diameter, preserving speedometer accuracy and avoiding clearance issues.
• Seasonal Tire Changes: When swapping between summer and winter tires, you can compare their dimensions to ensure they fit your vehicle correctly and do not affect your odometer or speedometer readings.
• Replacing a Damaged Tire: If your original tire size is unavailable, compare alternative sizes to find the closest match for safe and balanced vehicle handling.
• Optimizing for Performance or Comfort: By evaluating sidewall height and overall diameter, you can select tires that enhance ride comfort, handling, or fuel efficiency according to your driving preferences.

Advanced Tips & Best Practices

• Always keep diameter difference within 2-3% of the original tire to avoid issues with ABS, traction control, and speedometer accuracy.
• Double-check your vehicle’s factory-recommended tire sizes, often found on the driver’s side door jamb or in the owner’s manual, before making changes.
• Remember that a lower aspect ratio (shorter sidewall) generally improves handling but can reduce ride comfort and increase risk of rim damage on rough roads.
• Use the revolutions per mile metric to estimate impacts on your odometer and trip computer readings for accurate mileage tracking.
• When considering wider tires, ensure they clear suspension components and fenders, especially when turning or under load, to prevent rubbing or damage.

Frequently Asked Questions (Optional)

Will a small change in tire size affect my speedometer?

Yes, even small differences in tire circumference can cause your speedometer and odometer to display inaccurate readings. The calculator’s speedometer error output helps you identify and minimize this effect.

How much diameter difference is safe when changing tire sizes?

It’s generally recommended to keep the overall diameter change within 2-3% of the original tire to maintain proper vehicle operation and safety systems. Larger differences may impact handling, braking, and electronics.

What if my new tire size is not listed in my car’s manual?

Always prioritize manufacturer recommendations, but if you must choose a different size, use the calculator to find the closest match in diameter and circumference. Consult a tire or automotive professional to ensure safety and fitment.