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Specs & ConversionsTire Size Calculator — Compare Sizes & Speedometer Error
Thinking of changing tire or wheel size? Enter two sizes in width / aspect ratio / wheel diameter form. We’ll compute each tire’s overall diameter and the speedometer error the swap would cause.
How this calculator works
A tire’s overall diameter is the wheel diameter plus twice the sidewall height, where sidewall = width × aspect ratio. We convert both sizes to inches and compare. If the new tire is taller, your speedometer reads slow (you’re going faster than it shows); shorter, and it reads fast.
What changes the number
- Keep overall diameter within about ±3% of stock to avoid speedometer error, rubbing, and ABS/traction quirks.
- A “plus size” (bigger wheel, lower-profile tire) can keep diameter constant while changing the look and ride.
- Wider tires can rub on fenders or suspension and may need a different wheel offset — check fitment, not just diameter.
Frequently asked questions
How do I read a size like 225/45R17?
225 is the tread width in mm, 45 is the sidewall height as a percent of that width (the “aspect ratio”), and 17 is the wheel diameter in inches. Enter those three for each tire.
Will bigger tires hurt my speedometer?
A taller overall diameter makes the speedometer read slower than your true speed, and shorter reads faster. This calculator shows the exact percentage so you know what to expect.
How much size change is safe?
Staying within ±3% of the original overall diameter is the common rule of thumb. Beyond that you risk speedo error, clearance problems, and gearing changes.