Tire Size Calculator

Calculate tire dimensions from size string.

Inputs

Input Mode

Tire Size

About Tire Sizes

Understanding tire sizes helps in selecting replacements and understanding speedometer changes.

Format Example

P225/65R17

225: Width in mm
65: Aspect Ratio (%)
17: Rim Diameter (inches)

Results

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How Tire Size Calculation Works

Diameter = (2 × Section Width × Aspect Ratio / 25.4) + Rim Diameter | Circumference = π × Diameter

Tire sizes use a standardized notation (e.g., 225/45R17). The first number is width in millimeters, the second is aspect ratio (sidewall height as % of width), and the third is rim diameter in inches. This calculator converts these to actual dimensions.

1
Enter Tire Size
Input tire notation like 225/45R17
2
View Dimensions
See calculated width, height, diameter, and circumference
3
Compare Sizes
Optionally enter second tire size for comparison
4
Check Fitment
Review speedometer error and clearance impact

Use Cases

Size Comparison

Compare dimensions when considering alternative tire sizes

Speedometer Accuracy

Calculate speedometer error when changing tire sizes

Fitment Planning

Ensure new tires will fit without rubbing

Plus Sizing

Calculate correct tire size when upgrading wheel diameter

Tips

1
Stay within 3% of overall diameter to maintain accurate speedometer
2
Larger diameter tires will make speedometer read slower (actual speed is higher)
3
Lower profile tires (lower aspect ratio) provide better handling but harsher ride
4
Wider tires may rub on suspension or fenders when turning
5
Always verify fitment with manufacturer specifications or fitment guides

Common Mistakes

Ignoring speedometer error when changing tire sizes
Not checking clearance for width changes, especially on lowered vehicles
Assuming same aspect ratio means same diameter (it doesn't if width changes)
Forgetting that load rating and speed rating must meet or exceed OEM specs

Frequently Asked Questions

What do tire size numbers mean?

In 225/45R17: 225 is width in mm, 45 is aspect ratio (sidewall is 45% of width), R means radial construction, 17 is wheel diameter in inches. A taller aspect ratio means more sidewall.

How much can I change tire size?

Stay within 3% of original overall diameter to avoid significant speedometer error and clearance issues. Width changes depend on available wheel and fender clearance.

What is plus sizing?

Plus sizing uses larger wheel diameter with lower profile tires to maintain overall diameter. Going from 16" to 17" wheels requires shorter sidewall tires. This improves handling but ride quality may suffer.

Will bigger tires affect my car?

Larger diameter tires: slower acceleration, higher top speed, speedometer reads low. Wider tires: better grip, worse fuel economy, potential rubbing. Always maintain load and speed ratings.

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