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Tire Size Speedometer Error 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 focused on one specific question: if I change tire sizes, how far off will my speedometer be? Enter your current tire's code and the new size you are considering — metric (225/45R17) or flotation (33x12.5R15), in any combination — and the tool computes the exact answer from the two overall diameters.

A speedometer is calibrated to convert wheel rotations into a speed reading using the diameter of the tire it shipped with. Fit a physically larger tire and the wheel turns fewer times to cover the same distance, so the speedometer — still counting rotations the old way — reads slower than you are actually going. A smaller tire does the opposite: more rotations per mile, so the display reads faster than reality.

The result panel states the direction and size of the error as a clean percentage, alongside the full diameter, circumference and revolutions-per-mile breakdown for both tires, so you can judge whether a plus-size or off-road tire swap needs a compensating gear or ECU recalibration.

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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?