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حساب کیسے کام کرتا ہے
Tire diameter is calculated from the size code: (section width in mm × aspect ratio ÷ 100 × 2 ÷ 25.4) + rim diameter in inches. That converts the sidewall height (doubled, for top and bottom) from millimeters to inches and adds it to the wheel diameter.
Your speedometer reads based on the ORIGINAL tire's rotations per mile, not the new tire's. Fit a larger-diameter tire and it covers more ground per revolution, so the speedometer under-reads actual speed — you're going faster than it shows. Fit a smaller tire and it over-reads.
Regearing — installing a numerically higher (lower gear) axle ratio scaled to the tire diameter change — restores the engine RPM at any given road speed back to what it was before the tire swap. That, in turn, restores the accuracy of factory-calibrated systems tied to that RPM-to-speed relationship, such as shift points and cruise control behavior.