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🛞 Tire Size Calculator

Compare two tire sizes for diameter, circumference, revolutions per mile, and speedometer error.

🛞 Original Tire
🛞 New Tire
📊 Comparison
Diameter Difference
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Original Diameter
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New Diameter
—
Original Revs/Mile
—
New Revs/Mile
—
Speedometer Reading at True 60 mph
—
🛞

Enter both tire sizes, then click Calculate

Guide

About the Tire Size Calculator

Last updated: August 2026 · Reviewed by the NeftCal editorial team

This free tire size calculator compares two tire sizes — like your current tires against a new "plus-size" option — to show the difference in overall diameter, circumference, revolutions per mile, and the resulting speedometer error, using the standard tire size notation printed on every tire's sidewall.

How It Works

Enter the width (mm), aspect ratio (%), and rim diameter (inches) for both your original and new tire, exactly as printed on the sidewall (e.g., 225/50R17). The calculator computes each tire's sidewall height, overall diameter, and circumference, then compares the two to show the percentage difference and how it would affect your speedometer's accuracy at a given true speed.

Who Should Use This Calculator

Anyone considering "plus-sizing" their wheels (larger rims with shorter sidewalls), drivers fitting a different tire brand or size than the factory original, and off-road enthusiasts checking how larger tires affect speedometer and gearing accuracy.

Real-World Applications

  • Checking whether a new wheel/tire combination stays within a safe overall diameter difference
  • Estimating speedometer error after fitting non-stock tires
  • Comparing revolutions per mile between two tire options for gearing considerations
  • Verifying tire size compatibility before buying new wheels
  • Understanding why a speeding ticket's radar reading might differ slightly from an odometer with non-stock tires

Tips for Accurate Results

  • Keep the overall diameter difference within about 3% to avoid meaningfully affecting speedometer accuracy, ABS calibration, and ground clearance.
  • Always read tire size numbers directly off the sidewall — don't estimate or round them.
  • Remember a larger new tire makes your speedometer under-read your true speed, while a smaller one makes it over-read.
  • Fit matching tire sizes on both wheels of the same axle to avoid drivetrain strain and handling issues.
Formula

How Tire Diameter and Speedometer Error Are Calculated

From sidewall notation to speedometer accuracy

Step 1 — Sidewall Height
Sidewall Height (mm) = Width (mm) × (Aspect Ratio ÷ 100)

Step 2 — Overall Diameter
Diameter (mm) = (Rim Diameter [in] × 25.4) + (2 × Sidewall Height)

Step 3 — Circumference & Revolutions Per Mile
Circumference = Diameter × π; Revs/Mile = 63,360 ÷ (Diameter [in] × π)

Step 4 — Speedometer Error
True Speed = Indicated Speed × (New Diameter ÷ Original Diameter)
📏

Three Numbers Define the Tire

Width, aspect ratio, and rim diameter together fully determine a tire's overall size and diameter.

🔄

Diameter Drives Revolutions

A larger diameter means fewer wheel revolutions per mile, and vice versa — this is the basis of the speedometer error.

🎯

Speedometers Assume a Fixed Diameter

Factory speedometers are calibrated for the original equipment tire's revolutions per mile — change the diameter and the reading shifts.

⚙️ Why This Formula Works

A vehicle's speedometer doesn't measure speed directly — it counts wheel (or driveshaft) rotations per unit time and converts that to a speed using the assumption of the original tire's circumference. Since distance traveled per revolution equals the tire's circumference, any change in tire diameter directly and proportionally changes how much actual distance corresponds to each speedometer reading.

🎯 When to Use It

  • Comparing your current tire size against a new or "plus-size" option
  • Estimating speedometer error after a tire size change
  • Checking revolutions per mile for gearing considerations

📋 Assumptions

  • Both tire sizes are entered using standard metric sidewall notation (width/aspect/rim)
  • No significant tire deflection or load variance is factored in beyond the nominal size

⚠️ Limitations

  • Doesn't account for tire deflection under load, which slightly reduces effective rolling diameter
  • Assumes standard metric tire notation, not older numeric or alphanumeric sizing systems
  • Doesn't verify wheel well clearance or rubbing — a physical fitment check is still needed
Walkthrough

Step-by-Step: How to Use the Tire Size Calculator

From sidewall numbers to a full comparison

Enter your original tire size

Type the width, aspect ratio, and rim diameter from your current tire's sidewall (e.g., 225/50R17).

Enter the new tire size

Type the width, aspect ratio, and rim diameter of the tire you're comparing against.

Click "Calculate"

See the diameter difference, revolutions per mile, and speedometer error instantly.

Example

Worked Example

A common plus-size upgrade

225/50R17 → 245/45R18

Original Diameter25.86 in
New Diameter26.68 in
Difference+3.18%
Step 1: Original: sidewall = 225×0.50 = 112.5mm; diameter = (17×25.4) + 2×112.5 = 656.8mm ≈ 25.86in.
Step 2: New: sidewall = 245×0.45 = 110.25mm; diameter = (18×25.4) + 2×110.25 = 677.7mm ≈ 26.68in.
Step 3 — Speedometer effect: At a true 60 mph, the speedometer (calibrated for the original tire) would read 60 × (25.86/26.68) ≈ 58.2 mph — under-reading your true speed by about 3%.
Diameter Change
+3.18%
Speedo at True 60mph
58.2 mph
Reference

Reading a Tire Size Code

Breaking down 225/50R17

Code SegmentMeaning
225Section width in millimeters
50Aspect ratio — sidewall height as % of width
RRadial construction
17Rim diameter in inches

Some tires also list a load index and speed rating after the size (e.g., 225/50R17 94V) — those don't affect diameter and aren't part of this calculator.

Use Cases

Practical Use Cases for the Tire Size Calculator

Where tire size comparison matters

🛞

Plus-sizing wheels

Check that a larger rim with a shorter sidewall keeps overall diameter within a safe range.

🚙

Off-road tire upgrades

Estimate speedometer and gearing effects of fitting larger off-road tires.

🔁

Tire brand switching

Compare a replacement tire's size against the factory original before buying.

⚡

Speedometer accuracy checks

Understand why an odometer or speedometer might read differently after a tire change.

⚙️

Gearing considerations

Compare revolutions per mile between two sizes for performance or towing setups.

Pros & Cons

Advantages and Limitations

What this calculator does well, and where it has boundaries

✅ Advantages

  • Free, instant, and requires no signup
  • Uses standard tire sidewall notation directly
  • Shows diameter, circumference, revs/mile, and speedometer error together
  • Helps catch an oversized or undersized fitment before buying tires

⚠️ Limitations

  • Doesn't account for tire deflection under load
  • Doesn't check physical wheel well clearance or rubbing
  • Only supports standard metric tire size notation
Reference

Common Mistakes and Expert Tips

Get an accurate comparison every time

❌ Common Mistakes

  • Assuming a larger rim always means a larger overall tire diameter — a shorter sidewall can offset it
  • Ignoring the cumulative diameter difference when changing both width and aspect ratio
  • Forgetting speedometer error direction — larger tires make it under-read, smaller tires make it over-read
  • Mixing different diameters on the same axle

💡 Expert Tips & Best Practices

  • Keep the overall diameter difference within about 3% for minimal speedometer and handling impact
  • Double-check physical wheel well clearance before committing to a size change
  • Recalibrate your speedometer if your vehicle supports it after a significant tire size change
  • Always fit identical tire sizes on both wheels of the same axle
📝

Summary: This calculator compares two tire sizes for diameter, circumference, revolutions per mile, and speedometer error — free and instant, using the standard sidewall size notation. Pair it with the Gas Mileage Calculator or Horsepower Calculator for other vehicle-related calculations.

FAQ

Frequently Asked Questions

Common questions about tire sizing

How do I read a tire size like 225/50R17?
225 is the tire's section width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width — here, 50% of 225mm), R means radial construction, and 17 is the wheel rim diameter in inches. Together these three numbers determine the tire's overall diameter.
How is tire diameter calculated from the size code?
Sidewall height (mm) = width × (aspect ratio ÷ 100). Overall diameter (mm) = (rim diameter in inches × 25.4) + (2 × sidewall height), since the sidewall appears twice — once above and once below the wheel. Convert the result from millimeters to inches by dividing by 25.4.
Why does changing tire size affect my speedometer accuracy?
Your speedometer is calibrated for a specific tire diameter's revolutions per mile. Fitting a larger-diameter tire means the wheel actually travels farther per revolution than the speedometer assumes, so your true speed is higher than what's displayed. A smaller-diameter tire has the opposite effect, causing the speedometer to over-read your actual speed.
What is a safe tire size difference when plus-sizing wheels?
Most tire professionals recommend keeping the overall diameter difference within about 3% of the original size to avoid meaningfully affecting speedometer accuracy, ABS/traction control calibration, gearing, and ground clearance. Larger differences can trigger a check engine light on vehicles that monitor wheel speed sensors for other systems.
Does tire size affect fuel economy?
Yes, to a degree. Larger, heavier tires with more rolling resistance can slightly reduce fuel economy, while a taller overall diameter changes effective gearing, which can also affect economy and acceleration. The effect is usually modest for typical plus-sizing but grows with more dramatic size changes.
Can I mix different tire sizes on the same axle?
Generally no — mismatched tire diameters on the same axle can cause uneven power delivery, strain on the differential (especially on AWD vehicles), and unpredictable handling. Always fit matching tire sizes on both wheels of the same axle, and ideally all four for AWD vehicles.
Learn More

Authoritative Resources on Tire Sizing

References to complement this calculator

🏛️ NHTSA — Tire SafetyOfficial US federal tire safety and sizing resource ↗ 🛞 USA.gov — Vehicle Safety ResourcesFederal consumer resources for vehicle equipment ↗

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