Tire Comparison Calculator

Swapping to a different tire size affects your car's speedometer, ground clearance, and handling — the Tire Comparison Calculator shows you exactly how. Enter your current tire size (width, aspect ratio, and rim size) and your new tire size to see the overall diameter difference. Secondary outputs include width difference, sidewall height difference, circumference difference, and speedometer error at 60 mph.

mm
%
inches
mm
%
inches

Results

Overall Diameter Difference

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Current Tire Diameter

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New Tire Diameter

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Width Difference

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Sidewall Height Difference

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Circumference Difference

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Speedometer Error at 60 mph

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Results Table

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Frequently Asked Questions

How is tire size calculated?

Tire size is calculated using three numbers: width in millimeters, aspect ratio as a percentage of width, and rim diameter in inches. The overall diameter includes the rim size plus twice the sidewall height (width × aspect ratio ÷ 100).

What to consider when changing tire size?

When changing tire size, consider the overall diameter difference, speedometer accuracy, clearance issues, load rating, and how it affects handling, ride comfort, and fuel economy. Keep the overall diameter within 3% of the original for best results.

How does tire size affect speedometer accuracy?

A larger overall diameter will make your speedometer read slower than actual speed, while a smaller diameter makes it read faster. The difference is proportional to the change in tire circumference.

What is the maximum safe tire size difference?

Generally, keep the overall diameter within 3% of the original tire size. Larger differences can cause speedometer errors, clearance issues, and affect vehicle handling and safety systems.

How do I read tire size markings?

Tire size is marked as Width/Aspect Ratio R Rim Size (e.g., 235/65R17). Width is in millimeters, aspect ratio is sidewall height as a percentage of width, and rim size is in inches.

Does changing tire size affect fuel economy?

Yes, larger/heavier tires typically reduce fuel economy due to increased rolling resistance and weight. Wider tires also create more aerodynamic drag. The effect varies but can be 1-3% per inch of diameter change.