Enter codes in the standard format, e.g. 225/45R17
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A tire size code like 225/45R17 packs three numbers together: the tread width in millimetres (225), the sidewall aspect ratio as a percentage of that width (45%), and the wheel rim diameter in inches (17). Multiply the width by the aspect ratio to get the sidewall height, double it (top and bottom), add the rim diameter converted to millimetres, and you have the tire's overall rolling diameter.
That overall diameter is what your speedometer and odometer are calibrated against. Fit a tire with a larger diameter and your car will physically travel further per wheel rotation than the speedometer assumes, so both your displayed speed and your odometer will read slightly low — and a smaller-diameter tire does the opposite.
As a rule of thumb, most manufacturers and tire shops consider a diameter difference of up to about 3% safe for most vehicles without needing to recalibrate anything or worry about clearance and gearing issues; larger differences are more likely to cause rubbing, speedometer error worth correcting, or handling changes.
The three numbers are: tread width in mm, sidewall aspect ratio as a % of that width, and rim diameter in inches. In 225/45R17, that's a 225mm-wide tire with a sidewall 45% as tall as it is wide, fitted to a 17-inch rim.
Your speedometer and odometer are calibrated to the original tire's rolling diameter. A different diameter means a different distance traveled per wheel rotation, so both readings drift by roughly the same percentage as the diameter change.
Many manufacturers and tire professionals treat up to roughly 3% overall diameter difference as generally safe for handling, clearance and speedometer accuracy, though you should always check clearance physically and consult a tire professional for your specific vehicle.
No — that page decodes one tire size at a time (sidewall height, diameter, revolutions per mile). This tool is for comparing two different tire sizes side by side, such as when considering a wheel upgrade.
