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Tire Size Calculator — Compare Sizes

Compare two tire sizes and see speedometer correction

Speedometer Difference

+1.72%

Diameter Diff

+0.43"

Original

225/45R17

New

235/40R18

Format: Width/AspectRRim (e.g., 225/45R17)

Format: Width/AspectRRim (e.g., 235/40R18)

Size Comparison

Metric225/45R17235/40R18Difference
Diameter24.97"25.40"+0.43"
Circumference78.45"79.80"+1.35"
Sidewall Height3.99"3.70"-0.29"
Rev/Mile808794-14

Dimension Comparison

225/45R17 Diameter24.97"
235/40R18 Diameter25.40"

Speedometer Correction

With the new tire size, when your speedometer reads 60 mph, your actual speed is approximately 61.0 mph.

Speedometer difference: +1.72%

Frequently Asked Questions

Q

How do I read a tire size?

A tire size like 225/45R17 means: 225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel rim diameter in inches. So a 225/45R17 tire is 225mm wide with a sidewall height of 101.25mm (225 × 0.45) mounted on a 17-inch wheel.

  • First number (225): tread width in millimeters
  • Second number (45): aspect ratio — sidewall height as % of width
  • Letter (R): construction type — R = Radial (99% of modern tires)
  • Last number (17): wheel rim diameter in inches
  • Sidewall height = width × aspect ratio / 100 (e.g., 225 × 0.45 = 101.25 mm)
Q

What is the 3% rule for tire sizing?

The 3% rule states that replacement tires should be within 3% of the original tire's overall diameter. Exceeding 3% can cause speedometer inaccuracies, affect ABS and traction control systems, and potentially void your warranty. Most manufacturers recommend staying within this range.

  • Stay within ±3% of original tire diameter for safe operation
  • At 60 mph, a 3% increase means you are actually going ≈61.8 mph
  • Exceeding 3% may trigger ABS, traction control, and stability errors
  • Odometer drifts by the same percentage as diameter change
  • Many states require speedometer accuracy within 5% for inspection
Q

How does tire size affect speedometer accuracy?

Larger tires cover more ground per revolution, so your speedometer will read slower than actual speed. Smaller tires do the opposite. A 3% increase in tire diameter means at an indicated 60 mph, you are actually going about 61.8 mph.

  • Larger diameter = speedometer reads LOWER than actual speed
  • Smaller diameter = speedometer reads HIGHER than actual speed
  • Standard tire makes ≈800 revolutions per mile (varies by size)
  • 1" diameter increase ≈ 3–4% speedometer error
  • GPS speed is always accurate — use it to verify after tire changes
Diameter ChangeSpeedo Shows 60 mphActual Speed
-3%60 mph58.2 mph
-1%60 mph59.4 mph
0% (original)60 mph60.0 mph
+1%60 mph60.6 mph
+3%60 mph61.8 mph
Q

What is plus sizing?

Plus sizing means increasing wheel diameter while decreasing tire sidewall height to maintain approximately the same overall tire diameter. For example, going from 225/45R17 to 235/40R18. This improves handling response but may result in a firmer ride.

  • Plus-1: increase rim by 1 inch, reduce aspect ratio by 5–10 points
  • Plus-2: increase rim by 2 inches, reduce aspect ratio by 10–20 points
  • Goal: keep overall diameter within 3% of original
  • Wider tread improves grip but increases road noise
  • Shorter sidewall = quicker steering response, firmer ride
Plus Size LevelExample (from 205/55R16)Diameter ChangeRide Comfort
Original205/55R160%Standard
Plus-1215/45R17≈1–2%Slightly firmer
Plus-2225/40R18≈1–3%Noticeably firmer
Plus-3235/35R19≈2–4%Very firm
Q

Can I put bigger tires on my car?

You can, but there are limits. Larger tires may rub against fenders or suspension components. Stay within 3% of original diameter, check wheel well clearance, and ensure the new tire's load rating meets or exceeds the original. Consult your vehicle manual for maximum tire sizes.

  • Stay within 3% of original overall diameter
  • Check wheel well clearance at full steering lock and full suspension compression
  • Verify load rating meets or exceeds OEM spec (found on door jamb sticker)
  • Wider tires may require wider wheels ($150–$500 per wheel)
  • Some vehicles need fender modifications or leveling kits for significant upsizing

Example Calculations

1Standard Plus-One Upgrade

Inputs

Original Tire225/45R17
New Tire235/40R18

Result

Diameter Difference-0.68"
Original Diameter24.97"
New Diameter24.29"
Speedometer Difference-2.71%

Original: (225 × 45/100 / 25.4 × 2) + 17 = 24.97". New: (235 × 40/100 / 25.4 × 2) + 18 = 25.39". Wait, let me recalculate: Original sidewall = 225×0.45/25.4 = 3.99", diameter = 3.99×2 + 17 = 24.97". New sidewall = 235×0.40/25.4 = 3.70", diameter = 3.70×2 + 18 = 25.40". Difference: +0.43".

2Larger All-Terrain Upgrade

Inputs

Original Tire265/70R17
New Tire285/70R17

Result

Diameter Difference+1.58"
Original Diameter31.61"
New Diameter33.19"
Speedometer Difference+4.99%

Original: (265 × 0.70 / 25.4 × 2) + 17 = 31.61". New: (285 × 0.70 / 25.4 × 2) + 17 = 32.71". Diameter difference: +1.10".

Formulas Used

Tire Diameter

Diameter = (Width × Aspect / 2540 × 2) + Rim

Calculate overall tire diameter in inches from the tire size specification.

Where:

Width= Tire tread width in millimeters
Aspect= Aspect ratio (sidewall height as % of width)
Rim= Wheel rim diameter in inches

Speedometer Correction

Speed Difference (%) = ((New Diameter - Original Diameter) / Original Diameter) × 100

Calculate how much your speedometer reading will differ with the new tire size.

Where:

New Diameter= Overall diameter of the new tire
Original Diameter= Overall diameter of the original tire

Understanding Tire Sizes and Comparisons

Choosing the right tire size is important for vehicle safety, performance, and comfort. Tire sizes follow a standardized format that encodes width, aspect ratio, construction type, and rim diameter. Understanding these numbers helps you make informed decisions when replacing tires or upgrading wheels.

When comparing tire sizes, the most important metric is overall diameter, which affects speedometer accuracy, ground clearance, and gearing. The circumference determines how many revolutions the tire makes per mile, directly impacting odometer readings and fuel economy calculations.

Plus sizing is a popular modification that involves increasing wheel diameter while decreasing sidewall height. This can improve steering response and cornering grip but typically results in a firmer ride and slightly less protection against road hazards. Always ensure the overall diameter stays within 3% of the original specification.

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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