Tire Size Calculator

Compare two tire sizes and see detailed dimension differences including diameter, sidewall height, circumference, and revolutions per mile. Plan your tire upgrade with speedometer error analysis.

Current Tire

New Tire

Current Diameter

26.31"

New Diameter

26.86"

Current Sidewall

4.66"

New Sidewall

4.43"

Difference

+0.55"

Diameter Change

+2.08%

Speedometer Error (Faster)

2.0%

Current Circumference

82.66"

Current Revs/Mile

767

New Circumference

84.38"

New Revs/Mile

751

Understanding Tire Size Numbers

A tire size like P215/55R17 tells you everything about the tire's dimensions. The first number (215) is the section width in millimeters — the widest point of the tire from sidewall to sidewall. The second number (55) is the aspect ratio — the sidewall height as a percentage of the width. A 55 aspect ratio means the sidewall is 55% of 215mm = 118.25mm tall. The R means radial construction, and the last number (17) is the rim diameter in inches. The total tire diameter = (sidewall height × 2) + rim diameter.

Common Tire Sizes

VehicleWidthAspectRimDiameter
Compact185mm6515\"24.5\"
Sedan215mm5517\"26.3\"
SUV235mm6518\"30.0\"
Truck265mm7017\"31.6\"
Performance245mm4019\"26.7\"
Sporty225mm4518\"26.0\"

Frequently Asked Questions

How do I read a tire size (P215/55R17)?
Tire sizes like P215/55R17 break down as: P = passenger vehicle, 215 = section width in millimeters, 55 = aspect ratio (sidewall is 55% of width), R = radial construction, 17 = rim diameter in inches. The sidewall height is 215 × 0.55 = 118.25 mm (4.66 inches). Total tire diameter = (sidewall × 2) + rim diameter.
What is tire plus sizing?
Plus sizing means increasing wheel diameter while keeping the overall tire diameter roughly the same. For example, going from a 17" to 18" wheel with a lower-profile tire (lower aspect ratio). The goal is to maintain the same speedometer calibration while improving handling and appearance. A good rule is to keep total diameter within 3% of the original.
How does tire size affect speedometer accuracy?
Changing tire diameter affects speedometer accuracy because the speedometer measures wheel rotations, not actual speed. A larger tire covers more distance per revolution, so actual speed is higher than indicated. For every 1" increase in diameter (about 3-4% larger), actual speed is about 3% higher than what the speedometer shows.
Can I put wider tires on my car?
You can typically go 10-20mm wider than stock, but check for clearance issues with the suspension, fenders, and wheel wells. Wider tires improve grip and handling but may reduce fuel economy, increase road noise, and affect speedometer accuracy. Always ensure the load rating and speed rating meet or exceed your vehicle's requirements.
What does the load index and speed rating mean?
The load index (e.g., 91 = 1,356 lbs per tire) indicates maximum load capacity. The speed rating (e.g., H = 130 mph, V = 149 mph, W = 168 mph) indicates maximum safe speed. Always use tires with at least the original equipment load index and speed rating for safety.
What's the ideal tire pressure and how does size affect it?
Tire pressure should follow the vehicle manufacturer's recommendation (found on the door jamb sticker), not the tire's maximum pressure printed on the sidewall. Larger or wider tires may require slightly different pressures. Always check pressure when tires are cold for accurate readings.

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