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Tire Size Calculator
Compare two tire sizes and see how diameter, width and sidewall height change — and what that means for your speedometer, clearance and fitment.
Compare tire sizes
Enter two sizes in width / aspect ratio / rim diameter (e.g. 225/45 R17) to compare overall diameter and speedometer change.
Results
- Current diameter
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- Current circumference
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- New diameter
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- New circumference
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- Diameter difference
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A difference over roughly 3% can affect speedometer accuracy, clearance and drivetrain behavior. Confirm fitment, load rating and clearance with the tire and vehicle manufacturer before changing sizes.
How to use this tool
Enter your current size and the new size you are considering, then compare the results. The format is width/aspect-ratio and rim diameter, such as 225/45 R17.
- Enter your current tire size in the first set of fields.
- Enter the size you are considering in the second set of fields.
- Review the diameter, width and speedometer difference, and keep the overall diameter change small to avoid clearance and reading errors.
Going from 205/55 R16 to 225/45 R17 changes the width and rim but keeps the overall diameter close, so the speedometer stays accurate while the tire gets wider and slightly shorter in sidewall.
What the results mean
Overall diameter determines whether your speedometer reads correctly and whether the tire clears the wheel arch and suspension. A larger diameter makes the speedometer read slow; a smaller one makes it read fast. Width affects steering feel, hydroplaning resistance and clearance against suspension components.
How it is calculated
Sidewall height equals width times aspect ratio divided by 100. Overall diameter equals the rim diameter plus twice the sidewall height, converted to a single unit. The percentage difference in diameter predicts the speedometer error.
Important limitations
This calculator cannot confirm that a tire physically fits your vehicle. Clearance, load index, speed rating, rim width range and manufacturer approval all matter, and a shop should verify fitment. Never exceed the load and speed ratings your vehicle requires.