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Tire Size Comparison Calculator

Parse metric (225/45R17) or flotation (33x12.5R15) tire codes, get sidewall height, overall diameter, circumference and revolutions per mile/km, and compare two sizes with the resulting speedometer error.

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Parsing tire sizes
Computing diameters
Comparing tires

Result

This page opens the tire size calculator ready to compare any two tires against each other. Type the code for your current tire on the left and the size you are considering on the right — metric codes like 225/45R17 and flotation codes like 33x12.5R15 both work, and you can even compare one of each.

The tool converts both sizes to the same set of measurements — sidewall height, overall diameter, circumference, and revolutions per mile and per kilometer — so a low-profile metric tire and a tall flotation tire land on a common, comparable scale. A to-scale diagram shows the rim and the full tire outline for both sizes next to each other.

Below the numbers, the comparison block spells out the diameter difference in inches and percent, and translates that into a speedometer reading error: fit a taller tire and your speedometer will under-read your real speed; fit a shorter one and it will over-read. Everything is calculated locally — no size or vehicle data leaves your browser.

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Executed in Browser

Your files and inputs never leave this device. Processing runs completely offline. 100% Secure.

Accepted Formats & Rules

  • Sidewall height, overall diameter and circumference for each tire, shown in millimeters and inches
  • Revolutions per mile and per kilometer, plus wheel RPM at a reference speed you choose
  • To-scale side-by-side diagram of the two tires and their rims, and a signed diameter difference in inches and percent

Key Benefits

✓ Reads both common tire code formats — metric (225/45R17) and truck/off-road flotation (33x12.5R15) — and converts between them automatically for a fair comparison.
✓ Explains the speedometer error a size change causes in plain language: whether the new tire makes your speedometer read fast or slow, and by how much.
✓ Runs entirely in your browser with a real formula, not a lookup table — any valid size in either format works, not just a fixed preset list.

Use Cases

  • • Thinking about fitting a plus-size or off-road tire and want to know if your speedometer will still read correctly?
  • • Wondering what overall diameter your 225/45R17 tire actually is before ordering a replacement?
  • • Need to compare a metric tire code against a flotation size like 33x12.5R15 before a lift kit?