Find a tire's diameter, circumference, and revolutions per mile from its size code, or compare two tire sizes.
A code like 265/70R17 breaks down as: 265 = section width in mm, 70 = aspect ratio (sidewall height as a percentage of width), R = radial construction, 17 = wheel diameter in inches. Overall tire diameter = wheel diameter + 2 × sidewall height, where sidewall height = width × aspect ratio.
A speedometer is calibrated for a specific tire diameter (via wheel rotation count). Fitting a different-diameter tire changes how far the car actually travels per wheel rotation, so the speedometer — and odometer — read slightly off from the true speed and distance.
Not necessarily — a wider tire with a lower aspect ratio (shorter sidewall) can end up with a similar or even smaller overall diameter than a narrower tire with a taller sidewall.
Often yes, as long as the resulting tire's overall diameter stays close to the original — this is called "plus sizing." Always check clearance and load rating too.
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