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Tire Size Calculator

Compare tire dimensions and check compatibility.

Last updated: June 13, 2026

Calculator

Current Tire Width

Current Aspect Ratio

Current Rim Diameter

New Tire Width

New Aspect Ratio

New Rim Diameter

Results

Current Tire Diameter26.7"

New Tire Diameter27.3"

Diameter Difference1.9%

Within acceptable range (±3%)

Speedometer Error+1.9%

At 60 mph, actual speed difference

How to Use This Calculator

1Enter your current tire dimensions: width (mm), aspect ratio, and rim diameter (inches).
2Enter the new tire dimensions you are considering.
3Compare diameters, difference percentage, and speedometer impact.

Formula Explanation

Tire diameter is calculated from the width, aspect ratio (sidewall height as a percentage of width), and rim diameter. The overall diameter determines speedometer accuracy and compatibility.

Sidewall Height = Width(mm) × (Aspect Ratio / 100) Overall Diameter = (2 × Sidewall Height + Rim Diameter × 25.4) / 25.4 inches

Variables:

WidthTire width in millimeters (e.g., 225)

AspectSidewall height as percentage of width (e.g., 55)

RimRim/wheel diameter in inches (e.g., 17)

Worked Examples

225/55R17 vs 235/50R18

1. Current: (225 × 0.55 × 2 + 17 × 25.4) / 25.4 = 26.74"
2. New: (235 × 0.50 × 2 + 18 × 25.4) / 25.4 = 27.28"

Result: Difference: +2.0% | Within acceptable range

Why Use This Calculator

Ensure new tires are compatible with your vehicle before purchasing.
Understand speedometer accuracy impact of different tire sizes.
Stay within the recommended ±3% diameter range for safety.
Compare multiple tire sizes quickly to find the best fit.

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Tips & Best Practices

Keep tire diameter within ±3% of original to maintain speedometer accuracy.
Wider tires improve grip but may rub fenders on some vehicles.
Going up one rim size usually means decreasing aspect ratio to maintain overall diameter.
Check your vehicle door jamb sticker for the manufacturer-recommended tire size.

Common Mistakes to Avoid

❌ Assuming any tire fits if the rim diameter matches.

✅ Width and overall diameter must also be compatible with your vehicle.

❌ Ignoring speedometer error when changing tire sizes.

✅ A larger tire makes your speedometer read slower than actual speed.

❌ Mixing different tire sizes on the same axle.

✅ Always use matching tires on the same axle for safe handling.

Frequently Asked Questions

How do I read tire size numbers?

In 225/55R17: 225 is the width in mm, 55 is the aspect ratio (sidewall height as % of width), R means radial construction, and 17 is the rim diameter in inches.

How much tire size difference is acceptable?

Most manufacturers recommend staying within ±3% of the original tire diameter. Going beyond this can affect speedometer accuracy, ABS, traction control, and transmission behavior.

Do bigger tires affect gas mileage?

Yes, larger and wider tires typically increase rolling resistance, which can reduce MPG by 1–3%. They also add weight, further impacting efficiency.

What does the aspect ratio mean?

Aspect ratio is the sidewall height expressed as a percentage of the tire width. A 55 aspect ratio on a 225mm tire means the sidewall is 123.75mm (225 × 0.55) tall.

Can I put wider tires on my car?

Usually yes, within limits. Going 10–20mm wider is generally safe. Too wide can cause rubbing on fenders or suspension components. Check fitment guides for your specific vehicle.

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Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.

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