Decode a metric tire size into its real dimensions, or compare two sizes for diameter and speedometer error — computed locally in your browser.
Tire Size Calculator Online
Decode a metric tire size into its real dimensions, or compare two sizes for diameter and speedometer error — computed locally in your browser.
The Tire Size Calculator runs entirely in your browser. The tire sizes you enter are never uploaded, and results are estimates — verify exact fitment with your tire shop or vehicle manufacturer before ordering.
The Tire Size Calculator decodes a metric tire size such as 225/45R17 into its real-world width, sidewall height, overall diameter, circumference, and revolutions per mile. Switch to Compare mode and enter a stock size alongside a replacement to see exactly how much bigger or smaller the new tire is and how far off your speedometer will read once it is installed. It is useful when you are shopping for a plus-size wheel-and-tire package, checking whether a size swap keeps the same rolling diameter, or figuring out why your odometer and GPS no longer agree. Built for drivers, mechanics, and tire shops who want the numbers before they order. Everything runs locally in your browser — no vehicle or purchase details are uploaded.
Features
Decodes width, aspect ratio, and rim diameter from a standard metric tire size
Calculates overall diameter, sidewall height, and circumference in inches
Shows revolutions per mile for gearing and odometer-accuracy checks
Compare mode contrasts two sizes for percent diameter difference
Estimates the resulting speedometer reading error after a size change
Accepts ZR speed ratings, load/speed index suffixes, and LT-prefixed sizes
Copy a plain-text summary of the decoded or compared results
Runs entirely in your browser with nothing sent to a server
How to use the Tire Size Calculator
Choose Decode one size or Compare two sizes
Type a tire size like 225/45R17 into the field
For Compare mode, enter both the original and the new size
Read the diameter, circumference, and speedometer-error results
Example
Input
225/45R17
Output
Diameter: 24.97 in
Common errors & troubleshooting
The calculator says it could not parse the size. — Use the standard metric format width/aspectR rim, e.g. 225/45R17. Load and speed index letters after the rim size (like 94V) are ignored automatically.
Comparing two sizes shows a 0% diameter difference unexpectedly. — Double-check both fields — a typo that produces the same width, aspect ratio, and rim will always compute an identical diameter.
The speedometer error sign looks backwards. — A positive percentage means the new tire is larger, so the speedometer under-reads (actual speed is higher than shown). A negative percentage means it over-reads.
Result doesn't match a tire shop's fitment chart exactly. — This is a size-math estimate from the printed sidewall numbers. Manufacturer tolerances mean actual diameter can vary by roughly 1-2%, so treat the output as a starting estimate — verify before ordering.
Frequently asked questions
What does the Tire Size Calculator do with a size like 225/45R17?
It reads 225 as the tread width in millimeters, 45 as the aspect ratio (sidewall height as a percent of width), and 17 as the rim diameter in inches, then computes the overall tire diameter, circumference, and revolutions per mile.
How does the Tire Size Calculator estimate speedometer error?
It compares the overall diameter of your original size to the new size. Since a vehicle's speedometer is calibrated to the stock tire's circumference, a larger replacement tire covers more ground per revolution and makes the speedometer read lower than actual speed, and a smaller tire does the opposite.
Can the tire diameter calculator handle ZR speed-rated and light-truck sizes?
Yes. It recognizes the Z in ZR speed ratings, ignores trailing load/speed index text like 94V, and accepts LT-prefixed light-truck sizes such as LT225/75R16.
What counts as a safe tire size comparison for a wheel upgrade?
Many shops recommend keeping the new overall diameter within about 3% of the original so the speedometer, ABS, and traction control stay reasonably accurate — use the comparison mode's diameter difference to check before you buy.
Does the Tire Size Calculator upload my vehicle or purchase details?
No. Every calculation happens locally in your browser. The tire sizes you type are never sent to ArrayKit or anyone else.
Why does revolutions per mile matter for a tire height calculator?
Revolutions per mile ties directly to your odometer and gear ratio math — a size change that alters revs/mile shifts both your displayed mileage and effective final drive ratio, which matters for towing, fuel economy, and GPS-versus-odometer discrepancies.
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