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Tire size calculator

Compare tire sizes by diameter, circumference and speedometer effect.

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Tire size calculator
Paste a tire size such as 205/55R16 and this tool works out its overall diameter, rolling circumference, section width and how many times it turns over per kilometer or mile. Add one or more sizes on the following lines to compare them against the first — the reference size, typically the tire your vehicle came with — and see exactly how much bigger or smaller each alternative is.
Every comparison reports the percentage difference in diameter, the resulting change in ground clearance (half the diameter difference, since the axle sits that much higher or lower), and — using the speedometer reading you enter — the actual speed you are doing when the dash shows that number. A speedometer is calibrated for one rolling circumference, so fitting a taller or shorter tire without recalibrating makes it read fast or slow by roughly the same percentage as the diameter change. Switch the tolerance threshold to flag any comparison that strays further than you are comfortable with — plus-sizing guides commonly recommend staying within about 3%, which is the default.
Turn on "Show alternative sizes within tolerance" and the tool searches a range of common production widths, aspect ratios and rim diameters for fitments whose overall diameter falls inside your threshold, closest first — useful when you are shopping for a size your current tire's manufacturer no longer stocks. Switch the unit option to see every figure in inches and mph instead of millimeters and km/h. Line endings from Windows or Mac paste in without a problem, and a size the parser can't read is flagged inline rather than stopping the rest of the comparison.
Everything runs locally in your browser — no size, vehicle detail or speed reading is ever uploaded. Copy the result, download it as a .txt file, or send the output back into the input to keep refining a comparison.
FAQ
What format does the tire size need to be in?
Metric format like 205/55R16: section width in millimeters, aspect ratio as a percentage, then R and the rim diameter in inches. A leading letter (P, LT) and a trailing load or speed index are both fine — they are ignored.
How is the speedometer error calculated?
It assumes your speedometer is calibrated for the first (reference) size. Actual speed is the indicated speed scaled by the ratio of the new tire's diameter to the reference diameter, which is the same percentage as the diameter difference.
What counts as ground clearance change?
Half the difference in overall diameter — since a larger or smaller tire changes the distance from the axle to the ground by its extra or reduced radius, not its full diameter.
Where do the alternative sizes come from?
From a range of widths, aspect ratios and rim diameters commonly sold by tire manufacturers. The tool computes each one's diameter and keeps only those within your tolerance threshold of the reference size.
Is my data uploaded anywhere?
No. Every calculation runs entirely in your browser — the sizes and speeds you enter never leave your device.