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Tire Size Calculator

Use this tire size calculator to compare tire dimensions before planning fitment, gearing, or speedometer changes.

Catalog QA baseline June 6, 2026Free toolSource note included

Live calculator

Tire size

Tire diameter26.74 in

123.8 mm sidewall height.

Revs per mile754

84.02 inches circumference.

Speedometer difference0.17%

Positive means the new tire is larger than the original diameter entered.

Tire size breakdown
PartEstimate
Sidewall height4.87 in
Overall diameter26.74 in
Circumference84.02 in

Quick answer

Tire Size Calculator: what it calculates

Tire Size Calculator calculates tire diameter from section width, aspect ratio, wheel diameter, and original tire diameter. The visible formula is Sidewall height = section width x aspect ratio / 100; overall diameter = wheel diameter + 2 x sidewall height.

ResultTire diameter
InputsSection width, Aspect ratio, Wheel diameter, Original tire diameter
FormulaTire diameter formula

Formula

Tire diameter formula

Sidewall height = section width x aspect ratio / 100; overall diameter = wheel diameter + 2 x sidewall height

Convert sidewall height from millimeters to inches before adding it to wheel diameter in inches.

How to use

Steps

  1. Enter section width in millimeters.
  2. Enter aspect ratio as the sidewall height percentage.
  3. Enter wheel diameter in inches.
  4. Add original tire diameter if you want speedometer difference.
  5. Review diameter, circumference, and revolutions per mile.

Example

Sample calculation

Tire size225/55R17
Sidewall height123.8 mm
Overall diameter26.7 in
Revs per mileAbout 756

Calculator use

Best for

  • Use this tire size calculator to compare tire dimensions before planning fitment, gearing, or speedometer changes.
  • Reviewing the visible formula and assumptions before relying on the tire diameter.
  • Comparing the output with the sample calculation and benchmark table before using it elsewhere.
  • Quick everyday math with the result and formula in one place.

Before relying on it

Check first

  • Using the tire diameter before confirming the visible inputs match the same task and context: section width, aspect ratio, wheel diameter, and 1 additional input.
  • Ignoring that convert sidewall height from millimeters to inches before adding it to wheel diameter in inches.
  • Skipping the source notes when the formula, benchmark, or warning depends on outside context.
  • Mixing units, dates, or original values across the same calculation.

Details

What to know before using the result

Section widthMillimeters

The first number in a common tire size such as 225/55R17 is section width.

Aspect ratioSidewall percentage

The second number is sidewall height as a percentage of section width.

Fitment boundaryVerify before buying

Actual mounted dimensions vary by brand, load, rim width, and vehicle clearance.

Source notes

References used for this calculator

NHTSAExplains tire size markings including nominal width, aspect ratio, and rim diameter.

Benchmarks

How to read the result

Within 1%: Close match.

Often keeps speedometer impact small, but clearance still matters.

1% - 3%: Mild change.

Check clearance, gearing, and speedometer effect.

3%+: Large change.

Needs careful fitment review before purchase or installation.

Calculator accuracy

Methodology and assumptions

Formula

Sidewall height = section width x aspect ratio / 100; overall diameter = wheel diameter + 2 x sidewall height

Inputs used

Section width, Aspect ratio, Wheel diameter, Original tire diameter

Limitations

Everyday results are quick planning checks. Unit choices, rounding, labels, measurements, local prices, and real-world constraints can change the final decision.

Catalog QA baseline

June 6, 2026. This date marks the catalog-wide automated and editorial QA baseline, not a tool-specific expert review.

Cite this page

Toolkit Shelf. Tire Size Calculator. Page version June 6, 2026. https://toolkitshelf.com/tools/tire-size-calculator

FAQ

Common questions

How do I read 225/55R17?

225 is section width in millimeters, 55 is sidewall height as a percentage of width, and 17 is wheel diameter in inches.

Does this guarantee a tire will fit?

No. It estimates tire dimensions. Verify clearance, load rating, rim width, and manufacturer guidance before changing tire size.

Why does tire size affect speedometer reading?

A different diameter changes how far the vehicle travels per wheel rotation, which can make the speedometer read high or low.